AIP subs are not more advanced than nukes, but they are cheaper and potentially quieter than nukes...far more useful if your goal is defend your local waterspace and never go far from base.
If you want to sprint for days or weeks at 30+ knots and rapidly relocate significant sea control and power-projection forces from one part of the world to another (essentially holding all oceans at risk), you need nukes. Same if you need to patrol a vast ocean bordered by aggressive nations.
If the US were to find itself at a stealth disadvantage due to AIP opponents I suspect the solution would be a nuke with an AIP system to allow it to shut down the reactor plant. Keep the long-range sprint, but then go slow and quiet once in the patrol area. Or maybe just go with high-end batteries and lots of them, essentially what we have now but enough battery capacity to run for a longer period of time.
We probably wouldn't need a new class, I suspect we could retrofit some AIP capability if we needed to badly enough.
Quantity of subs is always a tradeoff with other platforms and programs. Everybody agrees we can always use more subs but we also need other systems and there's only so much money to go around.