There is a difference between Collins and Manchin and the likes of Kirk and Begich.
The latter were elected only because they ran in landslide elections, these seats will be in danger regardless of whether or not they retire.
Manchin and Collins, on the other hand, or incredibly popular and aren't in danger in any way. However, these seats are almost assured to flip once they retire because the type of Democrat/Republican that it takes to win in these states simply doesn't exist anymore.
Less so with Collins. The MEGOP has some credible candidates lined up for this event, especially if Poliquin wins
Poliquin wouldn't play well statewide. Assuming he's elected this year (which is a big assumption considering the NRCC triaged him), he'd likely be gone in 2016 anyway.
