I'm sure you're aware of Apple's fairly disastrous entry into the world of Mac clones back in the day. What you're arguing now is essentially the same theory back then: clones would benefit them by increasing the marketshare for their OS. Instead, clones just cannibalized Apple's own sales and diluted their brand with a bunch of crappy knock-offs. I'm not sure why anyone would expect it to play out differently now.
Sure, it might be in your interest as a consumer to have choice in hardware, but I don't see how it's in Apple's interest to do so. They seem to be doing pretty well precisely because of the choices they've made to lock things down, not in spite of them.