XSX has 10GB of fast memory that would be used as VRAM, not 12GB (and they are using AMD which uses more memory compared to Nvidia that has better compression).
This distinction is very shaky since not all data residing in a PC GPU VRAM has to be in a "fast" pool either.
It is better to view both PS5 and XS pools as UMA despite them being non-uniform in access bandwidths.
The question of how much VRAM you'd need is down to how much RAM is being consumed by console OS and game's logic in multiplatform titles, and this is likely to be at 4 or less GBs for most projects.
Then you'd have to consider that PC versions will likely get more advanced RT implementations which will add to VRAM consumption on top of that.
Thus 8GBs will likely be enough only for pure console level settings - and even in such cases there may be games which will want more VRAM.
Doesn't mean that games won't run on 8GB GPUs of course but performance may be sub par.
I agree that 8GB is not ideal for 4k long term, but with 3070 I wouldn't worry about that, because I would be already limited by the GPU level of performance and lowering settings anyway, reducing VRAM requirements in the process, so why would I worry about VRAM limitation when I'm already limited by the performance at 4K?
Consoles don't really do 4K and it is highly likely that lowering settings to get a game to playable fps won't affect VRAM consumption much.
Note that both AMD and Nv are avoiding releasing desktop GPUs with less than 8GBs of VRAM.
They know that it is the bare minimum right now, and as any bare minimum it will likely become an issue pretty fast.
for example why would I worry that WDL use more than 8GB at 4K while not even 3080 can do 4k/60fps/RT at least without DLSS?
Because for every WDL there will be a REVIII or Deathloop or I dunno COD all of which aim at high fps while consuming a lot of VRAM.
Shading / tracing performance isn't really linked to VRAM consumption. And with consoles being comparatively slow in both while having 16GBs of RAM it is very likely that many multiplatform games will use that baseline going forward - less advanced shading, higher RAM consumption.
We're also in the middle of crossgen period (which will end eventually) and this helps with RAM consumption since most games are aiming at previous gen limits for now.