As supporting technologies improve and materials science marches on, fringe technologies can sometimes leap quite suddenly into the mainstream. For example: the Stirling-cycle engine. It was a suboptimal technology compared to steam and IC engines for nearly 200 years, until Sweden found a use for it in their Gotland-class submarines. As it turns out, subs built with free-piston Stirling engines can run even more silently than their nuclear-powered competitors:
Nuclear powered submarines can be as quiet. They aren't typically as quiet as they could be, because they have pumps to circulate coolant around the reactor spaces, and they typically have the prop being directly driven by a turbine (through a gear train).
Both of those have solutions: natural circulation, and turbo-electric drive. Natural circulation is seen on the Ohio class submarines[0], in which the heat gradient inside the reactor is enough to cause it to circulate without powering a pump to move it around. It only works well at low power, but if you're trying to make a good imitation of a hole in the water, that's enough. And it's not like the Gotland-class is any better at going fast.
Turbo-electric propulsion was seen on the USS Tullibee[1]. It definitely was an advantage, in terms of making the submarine quieter. But it had a disadvantage in being heavier than the machinery required for geared drive.
And the USN wants nuclear submarines. A Gotland had a pair of 75kW generators onboard for power/propulsion. A S9G reactor drives a 30MW pumpjet on a Virginia class. SSNs can go faster when necessary, and have more power available to run all their systems.
https://en.wikipedia.org/wiki/Gotland-class_submarine