Offering the option to disable telemetry means Microsoft won't get telemetry data from the intersection of users who 1. don't just roll with the defaults for everything (many UX studies have shown that this applies to only a tiny percentage of users), and 2. care enough about privacy to want to turn telemetry off (this number is likely tiny relative to the overall Windows 10 userbase considering how great the adoption rates of Windows 10 has been so far despite mandatory telemetry), and 3. use Windows 10 (a significant percentage of users in 2. have already avoided Windows 10 specifically because of the mandatory telemetry).

That intersection of users seems absolutely miniscule relative to the overall Windows 10 userbase, and in most cases the telemetry data they contribute is not going to be any larger than a margin of error in Microsoft's Windows 10 overall telemetry trends.

Nobody is going to argue that telemetry isn't useful for developers, but at some point you have to decide if it's worth it to force users into giving you telemetry data if it means driving potential users away to competitors and giving fuel to a very vocal minority of users that write unfavorable articles and comments on news sites, blogs, and social media at every opportunity because of this decision.