Reports of the demise of the republican party are premature, and while it would be good to have a vetoproof congress, we don't have a president elected yet. And nothing I've seen gives me any reason to think that McCain will lose easily, if he will lose at all.
While I agree with your premise of McCain's electability, I need to believe that bush and the Iraq war will hang around McCain's neck like a necklace in the fall. Obama, the presumptive Dem nominee may have baggage but McCain will not be able to overcome the record and legacy of bush. Just my feelings and hope of course.
Consider McCain's plight. He has become a bush lapdog in order to appease the neocons in gaining the GOP nomination but he may also fall to the anger of the electorate who see bush as a loser and the reason we are trapped in Iraq. McCain may win the battle but lose the war.