Capacity is definitely a problem for the 3070, but bandwidth does help with the memory swaps and keep the GPU fed. Ideally the 3080 would have a minimum of 16GB and the 3070 12GB and I think we'll see higher capacity with the refresh cards (Super/Ti whatever they'll call them)
The 3070s bandwidth is not the real issue though, because it is still fast enough to keep it fed, especially at 1440p. Even though the 3070's 448 Gbps is less bandwidth than the 2080ti's 616 Gbps of bandwidth, it still keeps up with the 2080ti just fine. So its bandwidth is sufficient to keep the 3070 fed already I think. But more importantly, settings like textures, will not overly affect performance when increased, as long as the GPU is already powerful enough (sufficient TMUs etc), has sufficient bandwidth (in this case 448 Gbps), and enough VRAM capacity (8gb in the 3070's case). And you can check online benchmarks to see just how little higher quality texture settings effect performance with sufficient VRAM to handle them, or test it yourself at home. So I think the 3070 is most definitely able to handle new games higher texture quality settings in 2-3 years time I think, but is the VRAM capacity enough to allow them?
Same thing we are seeing with the RX 480/580 8gb vs RX 480/580 4Gb in Doom Eternal. The 480/580 8gb do great at 1080p Ultra settings, thanks to that 8gb. But the 4gb models are restricted by the game itself to only use high settings at 1080p, unless you lower textures. So it is not because the 4gb models are not powerful enough, or have insufficient bandwidth to run Ultra settings, but because they have insufficient VRAM to allow Ultra settings, and Ultra textures specifically (textures will always be the most demanding VRAM wise, but other settings such as detail, draw distance etc, also do their bit to increase VRAM usage, as does raising resolution).
So if the 3070 had 16gb (or 12gb), it would be able to use settings such as the highest textures, that an increased VRAM capacity benefits tremendously, while also not adversely affecting performance in most cases when using said settings. More VRAM can also benefit all settings that increase VRAM comsumption of course, but textures is a prime example of one that uses lots of VRAM, but doesn't affect performance adversely while doing so, as long as VRAM capacity is sufficient. And if VRAM limitations come into play in the next 2-3 years, then you are lowering settings such as textures, but not because the 3070 is unable to play games with higher texture settings, but because VRAM capacity is limited.
But bandwidth does also have a roll to play, you are absolutely correct, but in this case, more VRAM is the correct solution, as the 3070 is already sufficiently fed to be able to use larger textures, and settings like textures will only continue to grow in VRAM requirements as next gen moves on. But hopefully we do see some higher VRAM capacity models like you mentioned. Nothing wrong with stimulated completion from AMD, to bring NVIDIA into an more competitive state, that will just benefit more consumers in the end. AMD pushes NVIDIA to be more competitive, and in return NVIDIA pushes AMD to be more competitive. Awesome, great for all consumers.
And once again, I also don't know when lower capacity will become an issue precisely, it could be 1, 2 or more years. And I don't want anyone to panic either, I am just trying to explain what the benefits of the higher capacity VRAM are, which would be longevity, by being able to use VRAM demanding settings in upcoming games.
