Sorry but your in fantasy land, Pc's cant get 2 GPU's working and SLI, you think Nintendo can do what Nvidia and AMD struggle with lol.
They will be hiding GPU's in power supplies next like Misterxmedia...
If there is stronger CPU + GPU, there will be a new board..which has on all the expensive components imo. There are something you could reuse like power supply, disk, modem, box and the like, but most of the value is the main board.
Maybe the SCD is a slot in GPU card....cant see it though, but there will be no $ 99 plug in box boosting power
There are so many reasons for what you mentioned. SLI sucks and multi discrete adapters as DX12 or Vulkan allows is superior. Using two distinct gpus is possible and it offers more power than two of the same gpus in that mode or SLI.
We don't see SLI is because it's a pain to program games for. You would be right not expect nintendo to fail considering they lack expertise yet if any big company wanted to the time is right to make middleware that can be injected in to either of the APIS I mentioned that make it easier. Games make SLI problematic, yet if any of the big 3 were to build it up naturally so that it can be used easier games would go in to another mega era of computational improvements.
AMD/Nvidia don't struggle with SLI they did their part. The people that failed are devs, but to be fair SLI is rarely used. However a console designed like I mentioned would shit on any other console we have had to date or could have from that point on.