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.