Maine is a small state, and small states like to keep their senators for ages to get the goodies senior congresspeople can bring. Perhaps this tendency will change as the national mood and budget hawkery kills off "earmark culture" - I think the sun has set on the Ted Stevens/Dan Inouye/Rob Byrd types for good.
The best Bellows can do is hope to outperform Allen. Not entirely beyond the realms of possibility - 2014 reflects a vastly more polarised electorate than 2008, with more "one party" voters - but still unlikely given the political weather and the weaker ( 
sorry Bellowes ) candidate.
I'd actually wager that Bellows will narrowly outperform Allen, partly due to the increased polarization you speak of but more so because Bellows presents a starker contrast to Collins than Allen did. There's a small but significant chunk of Democrats/Democratic-leaning independents voters who approve of Collins, enough to vote for her in 2008, and would vote for her again in 2014 under certain circumstances but are very excited by Bellows and her ideas despite understanding her inability to win. It's the same reason that Scott Brown likely would have beaten a more moderate/generic Democrat in 2012; most were casting a vote for Elizabeth Warren rather than against Scott Brown.
Not that it will be much different, but maybe 2-3 points. It has nothing to do with Bellows being a better candidate than Allen (because obviously she isn't).
