The risk of becoming really bad is genuine, but the Celtics (obviously) have a lot going for them that most bad teams in the past have not. In other words, Boston can dive down for a good pick without leaving the cupboard bare.
Teams that hold their best player out for the majority of the season due to injury or "injury" tend to bounce back more quickly. (David Robinson being held out the Duncan year, and Pierce in 2007, are two examples.)
In addition, having an unbalanced roster helps. You rebalance it when you really want to win.
More importantly, having a wheelbarrow full of valuable draft picks to swap or use, means the team will likely be good pretty quickly. This is not normal for poor teams; poor teams usually have thrown away future picks.
That's why the historical example of tanking teams not getting good quickly doesn't apply. Most bad teams don't bring back sidelined all stars, or have multiple unprotected picks from other clubs.
One more obvious thing: They've got some of the auxiliary pieces in place. (Minus Rondo) the Celtics aren't talentless, they're starless.