The problem here is that Xbox isn't Sony's only competition at all. They're also competing with smartphones, movies, tablets, television, and basically any other entertainment medium that takes time away from console games. So even without Xbox around, they still have to work bloody hard.
I don't really agree. Not that other entertainment devices/mediums are also competition, but more about how hard they'd have to work. There's limited overlap between mediums and device categories, and in many cases they will be complementary, rather than directly competitive. This is why for example Nintendo places a lot of focus on the handheld market, whereas Sony places more on the home console market. Competition for Nintendo in the handheld market is not at all similar to what they face in the home market. The PSP (and later, mobile phones) was the first time in a long time that Nintendo had been given any reason to be concerned within that market. I would argue that without Xbox, other devices like PCs aren't suitable contenders to actually apply the sort of pressure that keeps a market healthy for consumers. The average person simply isn't going to connect their PC to their living room television, or buy one for they're child's bedroom etc. There would still be Nintendo of course, but they've pivoted so far away from attempting to compete directly with either of the other two, that I feel they are only slightly more of a competitive factor than something like an Apple TV is. Both Sony and MS both clearly respond to each other as competition (the entire PS4 is pretty much a direct response to the 360), and I don't really see anyone that strikes as a suitable substitute at this point. The console market simply doesn't appear attractive enough for a new player that could actually stand to compete with Sony to make a go of it from here.
Other mediums especially are poor competition, when they're typically value-adds for the consoles themselves.
Did MS need a gaming division? If you agree that big companies need to grow and branch up, the answer is "yes". So which companies may have interest in branching up: Samsung, Apple, Amazon, some Chinese company. All very unlikely atm, because this market has yet to show that it can support three big players in the long run. But the moment a huge gap emerges, someone might see this as a big opportunity. Just look at Sony: Sega and Nintendo had gaming in their DNA, Sony not a bit, and seemingly out of nowhere they came and conquered.
Sony entered the console market whilst it was clearly an emergent platform. Microsoft only entered as a response to Sony doing so. The companies you listed are all occupied with the current emergent platforms (mobile, tablets, cloud services, etc), and Sony and MS are both looking at that transition for the future. Nobody's going to want to go through an OG Xbox baptism by fire generation for the current market.