To take this back to the OP, nothing is decided in the Republican party. What it really looks like, assuming that Trump continues to get a similar level of support, is that the Republicans will have a brokered convention -- good for someone like Rubio, likely very bad for Trump or Cruz.
After Iowa and New Hampshire, though, it is too early to call what might happen. It likely depends on if Republicans can settle on a single "anti-Trump" candidate. The thinking is that won't happen, that you'll likely end up with at least three candidates; Trump and Cruz will keep battling each other for the "anti-establishment" candidate, and we'll end up with a single "establishment" candidate running against them. They'll split the vote, no candidate will get 50% of the delegates, and it will be decided in the convention.