Microsoft doesn't need to want to create a "botnet". NSA sure want to, and thanks to Snowden we know how close the ties between MS and NSA have gotten already. For instance, we know that NSA intercepts error reports sent by Windows machines to find vurnerabilities in the victim and infect them. Now they won't even need to wait for something to fail, they'll get it regularly. We can also just wait for them to order MS to increase the default telemetry setting, maybe bug something first and then increase the default explaining that it's to find the problem.
I don't have baseless trust in companies like most people apparently do. They're not there to make a good product, they're there to make a good profit. American ISPs are a good example of how these two things are not necessarily together. I'm not going to give companies information I don't want them to have if I can do something about it. Especially when we hear about security leaks every day. They don't necessarily need to exploit my information, but the hacker might find it useful.