Microsoft doesn't really have competitors in the traditional sense. Steam OS exists because they fucked up windows 8 so badly that Valve saw them as a liability to their revenue stream. Linux exists because hobbyists make it, and people use it out of preference. Apple is off doing their own thing.
Their biggest threat is shooting themselves in the foot so much that each leg is a bloody stump. I literally got pushed away from windows because they kept doing shit I didn't like, to the point I was willing to make the transfer, and eat the year or so it took to get comfortable with Linux. Linux is good, but to a majority of people it's the lesser evil at this point.
Absolutely, but the core development started as a personal project by Linus in 1991. The existence and ongoing maintaince of the product is owed to a collective effort of both individuals and corporate contributions, not a specific corporation looking to push a product. My point largely is that if Microsoft can't make a better problem than something that doesn't earn a guaranteed revenue stream, they largely have themselves to blame.