I think both options are possible, but a Big Rip like scenario seems more likely.
For a big crunch like scenario, that would need someone who has enough wealth and is also set out to destroy Bitcoin.
Even if someone like that doesn't buy 100% of all available Bitcoins (minus lost coins), if he buys a large enough majority of them, it would almost completely restrict the usage of Bitcoin.
A Big Rip like scenario is ofc the most likely, as you've explained above.
There will not be any big rip theory of bitcoin because bitcoin has got a big community behind, Bitcoin has no border and They consider bitcoin as a digital currency for purchasing, transferring and payment. Bitcoin will be central digital currency in the near future.