By this logic, though, 25 or 26 teams per season should just tank for the lottery.
We're close to a championship. Maybe we're not the #1 contender, but crazy things can happen. I mean, did anybody expect Dallas and OKC / Memphis to be the last three teams standing in the West? What if the Mavericks had decided to "blow it up" and trade Dirk, because their window was closed?
Here's the thing about rebuilding: it usually doesn't work. When we tried it the first time, our ceiling was the second round of the playoffs, with no real hope to go any farther. That's where we are now, except we had to endure years of misery and scores of losses to get there. It took us 20 years to get back to having a real contender.
Look at the Bulls as another example: their first attempt at failure was an utter failure. 15 years later, they're finally having some success.
So, in the case of the Celtics and Bulls, "rebuilding" took over a decade, and in each case required a near perfect storm (the Bulls winning the lottery, the Celtics being able to acquire two aging stars). For the vast majority of teams, they never even get that lucky.
Since, objectively, it's pretty unlikely that our rebuilding effort ever yields a team that is as good as the one we have now, I just don't see what the hurry is to lose 55+ games again.