They are not competitors. Netflix, Prime Video, Spotify... you don't buy movies/songs from them.
In XCloud you'll be able to play the games you own (and thus have bought) and it's when you buy things that the storefronts get their cut. See what happened with the Steam game streaming app and Apple
You're assuming all people think like you, and while I agree there might be some, there's also people that would rather play a game on PC with K&M but if there's the option on Xbox they might play them there instead, also, they plan to have a unified platform between Xbox and PC, that will never happen if there's no possibility of having K&M only games
If all this is because of Gears Tactics having K&M support, no one has ever said it won't have controller support too and I'm pretty sure it'll have because MS won't want the backlash because there's already plenty of examples in the genre (XCOM, Mutant Year Zero, Chroma Squad...) so they know people won't accept it.
Turn based strategy games are perfectly apt to be played with a controller, but real time no. AoE, Starcraft... those games would be infinitely frustrating with a controller (no, Halo Wars doesn't count because 1) it's infinitely better with K&M and 2) that's as simplified as it gets for a real-time strategy game, I'm talking "the full rts experience")
+1