Here's the thing: McCain is stuck with Palin.
If he throws her under the bus, his campaign is over anyway. The evangelical base would turn on him, and could not be placated without alienating nearly every other voter in the country. More importantly, McCain would have publicly admitted that his judgment had been unsound. It's bad enough that he's displayed poor judgment; he can't confirm that it was poor. The chief rationale for his candidacy would be gone.
And anyway, who in her or his right mind would step in to replace Palin? Who wants to go down in a losing campaign, and get tied to whatever debacle might be coming? The only people willing to take such a gamble wouldn't be an asset to the ticket. No one with a bright future wants to tie their reputation to a campaign that's self-destructing. As it is, I suspect that some people have already proved unwilling to run with McCain.
Add to that the problem that anybody who replaces Palin, except perhaps an actual Evangelical clergyman, will earn the undying enmity of her admirers in the Republican base. If she's thrown off the ticket, she will become St. Joan of the Mooseburgers, a martyr in the eyes of the cultural conservatives, and while McCain will be painted as Judas whoever he picks as a replacement will be seen as Barabbas. There's no future in that.