As long as people recognise thay a brand new CPU in a 5 year old board will have some compromises...
Which given the rates of #internetdrama these days, is probably not likely.
Doesn't seem like it (RTX 3090 is probably at the edge of what I'd consider "reasonably priced" anyway): https://www.reddit.com/r/hardware/comments/ikrteg/nvidia_rep...
The reason why PCIe 4.0 actually became a thing was because of enterprise storage arrays. M.2 slots and U.2 connectors don't have enough pins for 16 lanes, and using up so many of those lanes for one device makes no sense if you need to stick 10 or 20 of them in a server. That's also a use case that doesn't really make sense on AM4, unless you have a bifurcation[1]-capable motherboard or are spending way too much money on M.2 carriers with PLX chips in them.
[0] AFAIK, Intel wanted Thunderbolt direct-to-CPU but there was some weird driver/certification nonsense with Microsoft or something, and going through the chipset apparently made it easier for vendors not named Apple to support it. I don't remember the details.
[1] The ability to drive multiple PCIe devices off the same slot by splitting the slot's lanes. Most M.2 carrier boards are wired up to work this way because proper PCIe hubs are absurdly expensive... because the entire market for such chips are just storage array vendors.
Unfortunately we're still in something of a transition period. AMD blocked vendors from supporting PCIe 4.0 on 300/400 series boards that might have been capable of the required signal integrity (particularly on the first slot). AMD doesn't support PCIe 4.0 on their Zen3 APUs at all either - and some of the new processors are based on the APU die even though they don't have graphics, so they are limited to PCIe 3.0 as well. And obviously Skylake/Coffee Lake/Comet Lake stuff is all PCIe 3.0 based since they're ancient. So there are definitely scenarios where you might think "throw in a cheap dGPU" and are still stuck with PCIe 3.0.
Anyway though, what I would caution you here is, the 3090 has lots of VRAM, so it doesn't swap. What the 6500XT shows is, low-end cards can be more susceptible to PCIe bottlenecking - they have less VRAM, so they swap more, which increases the pressure on the PCIe bus. 3090 results are not representative of a worst case scenario just because they do drawcalls really fast (high framerate), there are other parts of the pipeline where PCIe load can be generated. If you are swapping due to low VRAM, that's still PCIe load.
Similarly - simply using a card placed into a PCIe 3.0x4 slot is not representative of Thunderbolt 3 results either, despite both links being 3.0x4. Thunderbolt is usually implemented as an additional standalone chip attached to the chipset - so it's not a CPU-direct link, there's multiple hops with higher latency there. There is also contention for the chipset bandwidth - the chipset has a 3.0x4 link, so the GPU alone can saturate it, but there's also NVMe traffic (particularly for pre-11th gen Intel where NVMe has to attach to the chipset) and network traffic (chipset provides the network controller), etc. It's bidirectional so you can read at 4GB/s while you write 4GB/s to the GPU, but there's also just general contention for commands/etc. So performance results on Thunderbolt will be worse than a card attached to the chipset, which will be worse than a card attached to CPU-direct PEG lanes, even if lane count is the same.
(the exception to this might be Ice Lake and newer Intel laptop chips, where the Thunderbolt controller is actually part of the CPU itself, the performance impact of that should be less. However, this does not apply to desktop chips, including Alder Lake.)
I was reading a techpower article on the 6500XT recently, because their power usage testing is excellent aand the 6500XT has an incredibly low idle draw versus other desktop cards.
PCIe 4.0 is one of those things that if you need it, then you already know you need it. But almost no one does.