Well, 3090 was poorly priced to begin with.
3080 was
$700,
4090 is 86% faster at $1600 aka about 129% more money. Not a good deal.
Maybe it looked good compared to
4080, because 4080 it was only 49% faster for 71% more money vs 3080, while 4090 vs
3090 was 63% faster for only 7%more money, but is a skewed comparison, because, as I've mentioned before, 3090 was a bad deal...
Of course, the elephant in the room, extra vRAM and DLSS3 which gives/will give extra performance on top for both compared to 3080, so it will sweeten the deal even more for 4080.
Right now, cheapest 4080 from where I usually buy is about 1260 euros. Cheapest 4090 is 1831 euros. 45% more money for about 25-30% more performance. Not really a great deal. Then add on top the possible need of a new PSU/case, a lot more heat and power used which you may cause a free sauna or the need for A/C etc.
So depends what your point of comparison is and how good is in itself. 3090 isn't great.