You “purists” really don't count, since it is human nature to ignore, completely, those who refuse to vote of to ignore those who go “Third Party”.
If you’re argument had any merit, then it would already be happening, since people have been *trying* to make it happen for years. Yet we still get stuck with Dole, McCain, Romney, and yes, Bush counts as well.
The GOP needs to make fundamental changes to the way it does business before your plan for winning it in the primaries actually would work. They’d need to close the primaries. They’d need to reform the primary schedule. But you and I both know that these things are never going to happen. The “leadership” in the GOP won’t allow it, even in the face of strong grassroots opposition - specifically because these changes would benefit conservatives to the detriment of the go-along-to-get-along crowd.
Ultimately, the only way the GOP can be saved is to make a clean sweep of its “leadership” caste - the hacks, the politicos, the backroom cigar smokers, the advisors - the whole lot of them. Any bets on how likely *that’s* going to be to happen?
We need a credible second party - one that basically exists in the shell of the old GOP, but which is actually responsive to the grassroots and which can bring back in the conservative-leaning independents and the Reagan Democrats. The current GOP simply isn’t, and will not ever again, get the job done.