I think GP is conflating Maths with Pure Maths. While I’m sure there is new ground to break in Pure Maths, I would agree that it’s getting harder and harder.

However, as you note, in Applied Maths there are more unsolved problems than you could address with a million researchers.

A physicist or pure Mather social looks at Navier-Stokes and says, “Oh, we know how that works, nothing to do there I guess”, whereas the applied mathematician looks at it and goes, “holy cow, this could keep my entire department busy for the rest of our natural lives”.