I'd go three years on a lower contract if need be. Getting him at 7 makes sense - and I think he can instantly be rejuvenated by moving to the bench and playing less minutes. I actually think the ideal move might be moving Pierce to the bench using him as a super 6th man.
Anyways even in rebuild mode - Ray Allen at 6-10 range is a good cnotract. Plenty of teams will be in a situation where (assuming the C's are done trying to contend) a Ray Allen could put them over the top. Basically old Pierce and old Ray Allen could be the billionaire's version of Posey and House on the 2008 team.
Let's say things fall apart next year, KG is not playing - etc. You could push Pierce in the kind of deal people suggested for Ray Allen (btw - I'd hate this with my heart, but just for thought exp.) Then you could move Ray Allen and Sheed or Ray Allen and Robinson (if they resign him for 4.

. Ray + Sheed is a managable trade at about 15 million annual, with a 1.5 years of sheed and 1.5 - 2.5 years of Ray Allen (and a half year lockout.)
You'd only have KG, Rondo (and presumably Perk.) Assuming KG recovers - he might even take a cheap buyout to go somewhere that matters after everyone is dumped. You'd tank the rest of the season in 2011 - and hope to get something good in the draft. Plus you'd be way under the cap even with holds etc. Not sure who the free agents are that season.
I think its pretty unlikely Ray goes elsewhere - he can pretty much only hope to get the MLE, so the list of places he'd go is probably only contenders with the MLE to spare.