Nothing stops them from doing both together. Put 8GB or 16GB on the APU package and then have a couple of SODIMM slots for more. The more wouldn't have the same bandwidth but it doesn't need to, because you're using it for browser tabs and filesystem cache, to keep that stuff out of the fast memory and preserve it for what needs it.

Moreover, they could put the APU in a socket and then if you wanted more of the integrated memory you could replace the APU without having to replace the entire machine.