Tanking is no guarantee of long-term success. Where The Celtics are now is no guarantee of long-term success. What leads to long-term success is making wise decisions in the position you are in. In this one upcoming draft The Celtics will have a lesser pick than if they had tanked. They still have all the assets they had before, the gazillion draft picks, the upcoming cap space. They still have Brad Stevens. They have IT which they wouldn't have otherwise had.
The point being, a winning franchise isn't made in only one way. To hold steadfast to the promise of tanking, to the belief that there is only one way to get to the top is shortsighted.
Think of it like moneyball---if everyone is tanking (and many teams were this year), tanking becomes less valuable. The odds of The Celtics getting a top lottery position where slim even if trying to fail as The 76'rs, Knicks and Lakers all failed way worse than The C's even could have. Tanking guarantees nothing.
The key is what you do from where you are, not just getting to the one place and thinking it will solve everything.