A 3090 isn't going to be "8K" capable in any demanding game, at least not natively without upscaling from far lower resolutions. 8K is equivalent to running four 4K screens at once, after all, so ignore any marketing nonsense calling it an "8K" card.
In reality, based on specs, a 3090 will only manage up to 20% more performance than a 3080, and at best maybe up to 75% more gaming performance than a 2080 Ti. The 3080, by comparison, is likely to be up to 50% faster than a 2080 Ti, though in both of those cases, I would expect the typical performance differences to be lower than that in most titles. And the 3070 will probably be around the performance level of a 2080 Ti, though it's hard to say exactly where it's performance will lie, and that will likely vary from one game to the next.
So, comparing the 3090 to the 3080, is it going to be worth spending more than twice as much for up to 20% more performance in games? Probably not, unless one has money to burn and is fine with paying around $800 more for a relatively small performance boost. It's basically a rebadging of what would have previously been marketed as a "Titan" card in the past. The announced pricing for the 3070 and 3080 seems relatively good, but arguably not so much for the 3090. The performance difference going from the 3080 to the 3090 is likely to be significantly lower than that of going from the 3070 (or 2080 Ti) to the 3080, but the price difference will be far higher.