I may very well happen anyway and thats why your argument is not valid.
I have found myself at odds with both parties and will not for for a candidate of either unless some very drastic changes are made.
Has it not occured to you that this is exactly what the supporters of the candidates I've listed were saying? Why did people consider Perot? Dems were disgusted or underwhelmed by Clinton, and conservative Republicans were ticked off over "Read my lips" and other stupidity on G.H.W.Bush's part.
I am as confident as I can be three years out that if Palin is the GOP nominee she will win going away and if she (or any other credible conservative) starts a third party meant to replace the GOP she will lose badly and so will the GOP candidate. In over 50 presidential elections since 1800, there is not one--not one!--where the plan you're talking about worked. People think it happened with Lincoln, but the GOP had already replaced the Whigs by the 1856 election.
We win when the GOP is a broad coalition between conservatives and moderates with conservatives holding the reins. If they leave that model aside (McCain) or the conservatives go elsewhere, the vote will split and the largest voting bloc will be committed Democrats.
Dealing with that reality and using it to our advantage does not require compromising a single principle.