This isn't really the natural evolution, but a forced evolution as a result of increased competition in the marketplace.
Microsoft placed a heavy emphasis on PS4 winning due to power differences. 360 won its gen for a long time due to performance differences as well as price and coming out earlier.
There is a very weird business mentality that coming out first, and having the best value, is what wins a generation. This is of course a fallacy because it's actually the games that win the gen, but nevertheless, both companies (sony and MS) knew that a generation was going to come sooner than normal, and so the reaction was to create a mid-gen console upgrade in order to stay competitive.
It sucks for console gamers to have to deal with something like this, but when you're up against a giant like MS, you gotta be prepared.
