Unlike the fashion world’s blink-and-you-miss-it product cycle, the high-end watch industry takes its, well, time with a trend. That explains why, after at least five years of sustained interest (and soaring prices), the vogue for steel sport watches with a 1960s and ’70s heritage continues unabated.

The mania — some have called it insanity — applies to both new and vintage timepieces, and it is especially pronounced for a handful of models manufactured by Rolex, Audemars Piguet and Patek Philippe. Many point to October 2017, when Phillips auctioned Paul Newman’s Rolex Daytona for $17.8 million (buyer’s premium included), as the start of the frenzy. But nothing compares with what’s happened to the category over the past two years.

Consider the current market value of the Tiffany Blue dial version of Patek’s highly coveted Ref. 5711 Nautilus, introduced in December for a retail price of $52,635. According to several dealers, the steel model is now rumored to be worth $3 million on secondary channels — to say nothing of the $6.2 million a buyer paid for a special edition auctioned for charity at Phillips New York in December.

The hype is causing a growing number of collectors to wonder: Has the category jumped the shark? (And if so, is another type of watch poised to replace it?) Or will steel sport watches maintain their hegemony over the luxury watch business?

The answer may be all of the above.

“There’s not necessarily a slowdown on blue-chip stainless steel sport models, but there’s room for others to come up,” James Lamdin, the New York-based director of vintage and pre-owned timepieces at the Watches of Switzerland Group, said in a recent phone interview. “I’m already seeing it and it’s going to get bigger and bigger: Smaller, thinner, precious metal timepieces with a more avant-garde design language, in both vintage and contemporary.”

Keith Davis, head of watches for Christie’s in the Americas, pointed to the house’s November live sale in Geneva, where all 119 lots sold. “The fact of the matter is, everything did exceptionally well,” he said, also in a phone interview. “There are spikes in value in areas that we didn’t see five years ago. It’s not about what’s next; it’s about a broadening.”

While cash-rich newcomers to watches continue to clamor for the steel classics, seasoned collectors seeking their next fix are looking at “what’s difficult to get, what’s limited in production, what’s likely to go up in value,” said Edouard Meylan, chief executive of the independent brand H. Moser & Cie.

A handful of independent brands — including Moser, De Bethune, MB&F, Urwerk, Laurent Ferrier and Voutilainen — are benefiting from the phenomenon Mr. Meylan described, a trend that has been well documented.

Less familiar is the attention some collectors are now devoting to dress watches, an industry term for watches designed to be worn with suits and at formal occasions. The interest is concentrated on dress styles that were manufactured from the late 1980s to the early 2000s by the watchmakers who laid the groundwork for today’s independent scene, including Daniel Roth, Roger Dubuis, Michel Parmigiani and Franck Muller.

In July, A Collected Man, a London-based website for rare watches and other collectibles founded by Silas Walton, posted an essay on “the rise of neo-vintage watches,” referring to timepieces that are not quite new, not quite vintage and have long been “discarded by collectors, precisely because they fell between the cracks.”

“People are recognizing these watches were made at a time when the watch industry was reinventing itself,” Mr. Walton said on a recent video call. “They were made to an extremely high level of quality; the design was very good in many cases. But once they were out of fashion, they disappeared into drawers and there was not enough activity to move the market until the age of Instagram and online platforms like ours, and suddenly people rediscovered them.”

Mr. Walton referred to a Roger Dubuis chronograph from the late 1990s certified with the prestigious Poinçon de Genève quality hallmark, made “by the finest case makers, with the finest movement, the finest finishing and yet it’s selling for $20,000,” he said. “How does this make sense?”

Equally undervalued, according to Aurel Bacs, the Geneva-based auctioneer who leads Phillips’s watch department, are “cool gentlemen’s watches,” which he described as thin, 35-millimeter styles from the 1960s and ’70s by brands such as Audemars Piguet, Piaget, Patek Philippe and Vacheron Constantin.

“You couldn’t give them away a few years ago because they didn’t give you that status,” he said.

Sean Song, a rare and vintage watch dealer based in Kuala Lumpur, Malaysia, said that Jaeger-LeCoultre’s famed Reverso model suffered from a similar problem, making it “hugely undervalued.”

It’s got “everything needed to make it a collector’s dream, with enough variety, complications and a strong heritage and design,” Mr. Song wrote in an email. “I particularly like the Reverso Chronograph Retrograde 270.2.69. Limited to 500 pieces and with a beautiful manually wound caliber, I don’t see why this isn’t at the levels of early Dubuis and Daniel Roth.”

Setting aside the Reverso’s rich, Art Deco-era history, the rectangular model may also be riding a wave of newfound interest in what are known as “shaped watches,” essentially any timepieces that are not round. Cartier dominates the space owing to its history of making watches with distinctive silhouettes — consider the elegantly elongated Tank, the bathtub-shaped Baignoire and the bent-out-of-shape Crash — but industry observers are seeing all kinds of shaped watches, from Patek’s groovy 1968 Golden Ellipse to Franck Muller’s tonneau-shaped perpetual calendars, rise in value.

Artemy Lechbinskiy, chief executive of Ineichen Auctioneers in Zurich, explained the growing interest in offbeat watches in general as a reflection of the market’s heat.

“Market prices of steel sport models are already so high that 90 percent of collectors and end customers are not ready to pay this amount,” he said on a recent video call. “If you have liquidity and you’re still passionate, you need to invest in something.”

Other once-mocked styles that have made a big comeback of late: gold and two-tone wristwatches, which many observers say has to do with the scarcity of the most coveted steel models.

“We see a huge demand for gold watches that is difficult to fill,” said Ruediger Albers, president of Wempe Jewelers on Fifth Avenue in New York City. “People love certain models and if they can’t get it in steel, they think they’ll treat themselves to the gold one, just to find that that one is difficult to get as well.”

The same phenomenon is also driving interest in lesser-known steel models with great design history and cult appeal, such as the 1955 Ingenieur from IWC Schaffhausen, the 1975 Laureato by Girard-Perregaux and the 222 model introduced by Vacheron Constantin in 1977, the predecessor to the brand’s long-running Overseas collection of sport watches.

On a recent video call, Gabe Reilly, co-founder of Collective Horology, a California-based collectors group, contemplated the market trends of the past two years — the shortage of key models, the skyrocketing prices, the cynicism — and sighed.

“It’s really de-motivating to find out you can’t buy the watch you want,” he said. “But there’s a silver lining: It’s causing people to look in other corners.”

Mr. Reilly spoke from personal experience. “I’m wearing a Girard-Perregaux Laureato,” he said. “It’s probably not a brand I would have looked at if it weren’t impossible for me to get an Audemars Piguet Royal Oak chronograph. But I discovered what is a better watch for me.

“We see other people discovering steel sport watches from independent brands, like Czapek,” he added. “And it’s also sending people in the direction of cheap and cheerful — taking solace in microbrands and enjoying collecting things at a significantly lower price point. It’s like a particle collider — I see the interest going all over the place.”