I hate to admit it, but this exactly right.
Outside of Beckett (and, to a lesser degree, Lackey), the Sox haven't made any big mistakes with their SP. But I think they've also learned from their mistake with Beckett, and that is to not sign pitchers on the wrong side of 30 to long term deals.
If the Sox do trade Lester, they'll have to get back a Shields-esque return (i.e. a top 20 prospect, close to the MLB).
I also wouldn't be surprised if the Sox decided to dish Lester, and then package that prospect up with another 1-2 high level guys (Cecchini, Webster, Barnes, type, or even Middlebrooks, if Bogaerts looks ready and they believe in Iglesias) in a deal for a guy like Stanton. Just depends on who they can get in return for Lester.