Another possible outcome is a mining cartel taking over and becoming a de facto central authority, with power to change the protocol, set minimum fees, cancel transactions and seize funds, etc.
21 inc is a fine candidate for that. No idea how they make the mining work, but I assume they know how.
There could be tens of millions of devices slaved to their pool and on top of that most of the people hashing away won't actually think through what they're doing.
The present miners are free to point their power anywhere else within a second. That would not be the case if 21 decided they wanted to have a try at 'improving' the protocol.