People have very strong expectations about what MS can and can't do with the non-UWP environment. If nothing else, there's a ton of legacy software there that they don't want to throw under the bus (and there'd be serious uproar if they somehow did). With UWP though, not only do people have much weaker versions of those expectations, but MS has already (wrongly) taken a sizable chunk of the PR hit they'd get for abusing it, and the remaining barrier is lower.
On vanilla Win10, I see nothing to worry about. Win10 for ARM, Win10S, whatever niche version comes next.... I dunno. If something sufficiently different like that somehow takes off, MS will have all kinds of opportunity to do "interesting" things with it, and messing with graphics APIs could be a critical force multiplier for many of those scenarios.
(I don't really have big problems with MS as a company these days, but their history is full of too many lock-in tactics, and I definitely still don't consider it safe to make plans that won't work right if they head back in that direction.)
synthtel2 wrote:If they really wanted to be obnoxious about this
To clarify: not saying that that action would be in itself the obnoxious one, but that that action would be a bigger tip-off if they're up to no good than anything that's actually happened so far.