Gaming-wise, I think the 3090 struggles at 8k and 4k 144hz gaming, which is very high end gaming experience and people that spend the money on those monitors will spend the money on the 4090.

For other purposes (like scientific computing/ml), while 3090 can do a lot of things that 4090 does, it's half the speed (or even slower) and a lot less energy efficient, say 1x4090 = 2x3090, 4090 uses 450W, dual 3090 uses 700W, of course you can power limit both of them further, but you still get best bang for your buck out of the 4090.