"I would of bought a XB1 but it had an external PSU, so I bought a PS4." is a justification thats probably never been used.
If it had its probably somewhere between. "because 4 is bigger than one" and "because I am Peter Simons the fourth" in terms of relevancy on the impact of overall sales.
If you had to eat 1+ billion in losses on your last product and you've only designed a handful of CE products, you might go with tried and true methods versus trying to get fancy.
Is someone really going to buy a PS4 because it has an internal PSU vs an external one? I can see a frequent traveller buying the PS4 because of the input voltage range, but the internal vs external thing is pretty much a stupid argument to have. Some people prefer one over the other, but I doubt anyone would would seriously consider it into their decision to buy either platform.
It may be shocking to some here, but companies can contract out design work. For a company like Microsoft, they don't really need to have internal talent for hardware. There are tons of capable people working for capable companies. Cheap and easy is sometimes desirable for companies, not because of lack of talent, but because it's ... well ... cheap and easy.