Your arguments about McCain and Romney were correct. It's the very same pragmatic argument many of us are making about Trump. Yet I'm missing how you got to your conclusion from your argument? Isn't it the "principled" no compromise #nevertrump group that took that position in '08 and '12? I usually see the linkage, but I'm more than slow today...
What is sauce for the goose is sauce for the gander.
One more thing.
I am a Republican because that is the party that's best represented my political positions, my beliefs and ideals.
I've voted for Republicans because I've found things in GOP candidates that I can identify with, admire or wish to emulate.
Donald Trump's Republican Party represents nearly none of my political positions, or at least those positions that I've managed to understand or that have stayed put long enough to grow some kind of roots. The same goes for my beliefs and ideals.
I don't, in the least, identify with Trump.
I detest the way that he's run his campaign and by extension, who he is, and I have no wish to emulate him in any way.
If Trump is a representation of what the Republican Party is today, then I'm not a Republican, and I have zero issues with not voting for the Republican candidate.