Don't the Xbox One and PS4 already have 8GB of RAM? Nintendo needs to move forward, not backward..
Yes and No.
From what I've heard, quite a chunk of that is reserved by the OS so that actual usable amount is less (up to 4GB last I heard although this may have increased since through OS updates).
If Nintendo are able to keep their OS to 512MB, in theory it may not be too bad for devs RAM wise.
Also, there are other factors to remember e.g. RAM speed, storage access speed (i.e. if it's on carts they may not need as much RAM for caching), etc.