Absent some additional impetus, I don't see McCain leaving the GOP. As the article indicates, in June 2001, he was unhappy with the direction the party was taking, but not ready to leave it. The likeliest PODs that could happen after June 2001 that might lead to McCain departing the GOP would be either 9/11 doesn't happen or the Bush Administration responds to it differently. Even then, I don't see him leaving until after the 2002 midterms, and maybe running as an independent for reelection to his Senate seat in 2004.
So before one can answer the question of what would happen if John McCain left the GOP, you first have to answer why he would leave the GOP.
I entirely agree, I think it’s too unrealistic of a question. But that doesn’t mean we can’t ponder it
