Radical leftist flamer is not a RINO
I view this new guy as a placeholder. If Tierney survives this challenge, he'll probably die in office, drooling and senile. If the new guy works out, we support him and he goes on to serve for as long as he can win elections, with GOP support. If he doesn't toe the line, we primary him and run against him if he switches parties. It's a lot easier to displace a 1-term incumbent than an 8-term incumbent like Tierney.
Irrelevant!
You don't know how legislative bodies are organized, do you?
Even if Tisei's views on policy were identical to Tierney's, it would still be preferable to have Tisei in the seat, since he is going to caucus with the GOP. That has impact on who the Speaker is and how the committees are organized. All of which has impact on What Is to Be Done!