First off, the Sox were still reasonably in contention at the ASB. Secondly, you think the team doesn't lose any fans when they're losing? Just take a look at the responses, or lack thereof, on this board. How many posters have said that they don't watch the games anymore? Thirdly, how many posts have we read stating that there is no reason why a team with this type of budget should be operating like a small market team? There is no reason why a team with this type of budget should finish in last place?
Boston's fans are more rabid and loyal than those of most teams. Unfortunately, they have recently become entitled. Whether people like to hear that or not, it's true. I can't see them being understanding and accepting of the idea of a complete rebuild.
Personally, I, myself, wouldn't like the idea of a complete rebuild. I agree that with the resources that this team has, there is no reason to have to go into a 3 or 4 year rebuild period. They should be able to contend every year, even when they are developing some kids.