From what I've read there's no reason to upgrade right now if you are on 6000 or 7000, 8000 is mainly to bump up AMD's marketshare. 9000 will see the return of the high end apparently.
Sounds exactly like the start of this GPU gen where none of the new GPUs were worth it at launch, with the older gen parts still knocking around giving better price-to-performance.
I'm not sure if AMD can even bump market share, as they don't seem to really be trying on the GPU front. Even taking into consideration there's no high-end next gen, doesn't have to mean that they aren't capable of releasing decent GPUs at decent prices. I was listening to a HWUB discussion on upcoming GPUs and they mentioned that AMD were really going for it and hungry to win, when it came to Ryzen over the past decade. But they don't show that same level of desire to win back marketshare when it comes to the GPU side of things.
Things can go two ways and we've seen AMD do both before:
1) rebrand/re-release or releasing new parts at fairly similar prices (i.e. little improvement to price to performance) such that it still feels like a re-release. E.g. 7800xt being barely better than the 6800xt for almost the same price.
2) just price the parts well. Mid-range for next gen should perform like top-range this gen at mid-range pricing. So if they price 7900XT(X) similar to 7700XT/7800XT (or ideally much better). AMD did do this before, back in the day they had 390X at the top end, which was followed up by a similar performing RX480 for far less money, actually at mainstream sub-£200 pricing. Even though AMD didn't have any high-end to compete in the RX400 series, the RX480 was still a win due to massive improvements to price-to-performance. I knew a fair few folks around that time were buying RX480s instead of whatever Nvidia had, especially since they were on a budget GPU-wise.
The former has been happening a lot recently from both AMD and Nvidia, though admittedly AMD has been seemingly doing it a lot more. It won't gain them much market share if they do that with the upcoming releases.
The latter is what AMD would do, if they really did care about regaining market share. Just price in a way that brings todays top end performance to the mainstream price-points. Performance wise, we don't know what AMD's next gen will top out at. But if similar to a previous time they didn't have high-end (RX 400), then they should ideally be giving us 7900XTX performance for 7700XT price.
The poor expectations I have makes me think we'll get 7900XT performance at 7800XT price, if we're even lucky to get that. But even that would at least be appreciated, over what we've had recently. Heck at this point I'll even take a £300 card performing like 7800XT if that's the best they give us.