If a 5700 xt does 50-52 mh at 125-130 at the wall
and the 3090 does 150 mh at. 230 at the wall
it simply does not have a huge edge. also I don't think it will do 150 at 230 watts.
more like 130 at 230 watts. but I consider buying one as soon as possible just to benchmark on my threadripper.
My 5700 XT Gigabyte does 54Mhs exactly, consuming 135 from the wall
Of course there's room for improvement for 3090 and Nvidia another cards with miners and drivers, but I don't see a huge leap to kill older generations of cards, since polaris it's almost the same, a good gain on hashrate, more efficiency, but not overkill cards
People still use polaris cards to mine
In my country will be difficulty to buy any new generation of cards with a reasonably good price