Trump may be a plurality candidate, overall, not a majority candidate, but there is no question that he’s the front-runner.
The nominating rules allow picking someone else, but they don’t mandate picking someone else. If the party picks someone else, it will not be due to popular will or demonstrated momentum. It will be due to a conscious decision to use the rules to override those things.
That won’t be illegitimate, per se, but it will be fatal. There is, in fact, no scenario in which the Republican Party continues to be what it has been during our lifetimes so far. It’s not the party of conservatism or small government if it nominates Trump. But at this point – in terms of probabilities, based on what we know – it falls apart if it doesn’t.