The question isn't how would they stop carriers from installing their own software, but why? Consumers continue to buy phones with carrier software, and I don't think anyone picked the iPhone instead of the Galaxy S5 because they were bothered by the inclusion of the T-Mobile MyAccount app (that can be disabled). If the inclusion of carrier software will get the carriers to advertise the phones for them I don't think the manufacturers are going to put an end to it.
No, they're definitely not likely to stop it because of getting their phones subsidised by the carriers (even though Apple does the same thing without carrier bloat).
But, as for the why? Because when Google comes out with an update to Android, consumers not only have to wait for the phone manufacturers to add their fingerprints (such as a custom launcher), but then there's a *longer* wait for the carriers to add *their* junk. So, for some consumers (like those on Verizon), Android could be a few revisions ahead before they get an upgrade to a now 'old' version. That doesn't happen with Apple, for one. When they make an update, *all* their (capable) devices get that upgrade as soon as its available.
Let's face it. Android 5 is out now, it'll be at least January before Samsung writes off on it, so likely February/March before most (non Verizon) carriers release an update. Will Android be up to 5.01 or higher by that point? How long then, before we get *that* update? That is is the
why Android needs fewer people (like the carriers) mucking with the software.