I think it's ludicrous you can't reinstall older versions of Apple's own OS versions. Apple could discourage it, make it a bit of a hassle, make it have big scary warnings first, sure, but I think they should provide a method for it for users who really want to do so once in full possession of the facts.
The features and experience (not to mention wider selection of new app releases/updates) of the newer OS versions should be compelling enough. People shouldn't ever want, or feel the need, to go back.
But this way, Apple gets a high adoption rate from almost everyone but the most wary of users and calls it a success, even if people end up hating it and wish they could go back.