Well the consoles also split that memory with what would traditionally be stored in RAM, so it isn't a case of 16 GB of VRAM being the obvious floor. I don't know what kind of split most games will end up using or what the baseline will be for titles in a few years, but it's fair to think that 8 GB is going to be necessary since even the Xbox Series S will have at least that much and there are a lot of older GPUs that have at least 8 GB which make it an easy target.
If we do end up needing closer to 16 GB in most games I don't think it will be because developers are optimizing for AMD hardware, but more likely because the consoles offer them that much and programmers tend to be lazy gits that will just use up whatever resources are available. Hence the old saying that software is a gas, since it will expand to fill the container (i.e., memory and other hardware) its stored in.
Personally I think we'll probably see something closer to 10 GB in the near and midterm because the Xbox Series X has 10 GB of fast RAM. Between whatever the operating system will take up while running in the background, that probably leaves enough memory for whatever games would typically need to store in RAM.