If NVidia had this capability all along I feel this begs the question of why they had to wait for AMD to implement it. Looks bad either way.
On the other hand, considering Ryzen's appetite for fast memory and the fact that NVidia uses GDDR6X, wouldn't it be an interesting day if it turns out Ryzen 5000 runs faster with an NVidia 3000 series card?
Yeah, it does seem a bit questionable why they wouldn't have enabled such functionality previously. Are there any drawbacks to doing so?
It's also possible that AMD's solution might cover more than just adjusting the PCIe BAR size though, and that might only be a part of it that affects existing games. I think it was suggested that games would have to be optimized for Smart Access Memory to get the most from it, so perhaps it also enables something like direct control over the contents of the "infinity cache" for example. Nvidia's performance gains from adjusting the BAR size alone might not be as large, though we just have speculation to work with for now.
As for Ryzen and RAM speed, that's not how it works. Ryzen's fabric matches the speed of system RAM, but isn't affected by VRAM, and applications are typically processing data stored in system RAM, not on the graphics card. And again, at least from what AMD has shown, the memory bandwidth of their 6000-series cards can effectively be far higher than GDDR6X for data that can fit inside the large 128MB block of L3 cache that they are calling the "infinity cache", which accounts for a relatively large portion of the GPU chip itself. That cache can hold the framebuffer, for example, allowing the GPU to perform operations on it much quicker and more efficiently than if it were stored in VRAM. I'm sure there will be some cases where having faster VRAM would be better, but this new cache is a large part of where AMD's performance and efficiency gains come from this generation, and it reduces the need for faster graphics memory.