Paul Pierce has been through it all in Boston. He was drafted in '98/'99 as the Celtics were in a rebuild. In his first few years, Rick Pitino was his coach, and the Celtics logged in losing seasons. Then Jim O'Brien took over for Pitino, and things started to improve in Boston. In his first full season as head coach, Brown led Pierce and the Celtics to a 49-33 record and an Eastern Conference Finals appearance.
From a losing record the previous season to the Conference Finals, Pierce felt he was moving into his prime, and the Celtics are ready to build on the success of that season and become a legitimate contender. But, Danny Ainge joined the Celtics as an executive and decided that the team reached its ceiling. In Ainge's mind, it was time for a rebuild despite the recent success. That didn't sit well with The Truth (via The Jump):
"I was like 'We're headed in a certain direction, we made it to the Conference Finals. Let's build on that.' For me, it (the rebuild) was very discouraging. I've started to hit my prime, and the team is regressing. We're rebuilding again. It started as a rebuild when I first got there. Then I'm in my prime, making All-Star teams, we get to the Conference Finals, and then we strip the team down again? That was the most discouraging thing for me; it took a lot of motivation out of me for a couple of years, I'm not even gonna lie about it."
The best parallel we can draw in today's NBA is Devin Booker. A talented player whose NBA career has been wasted in Phoenix so far. You can afford to tank for two, maybe three years. EVerything longer than that, and you start significantly limiting player development. No summer work and shooting drills can substitute competitive NBA games and playoff experience.
In addition to that, Pierce's experience shows that players just don't work as hard and sacrifice as much if their team isn't putting them in the best position to win. That time can never be made up for, and that human element is as crucial as draft projections when talking about tanking and rebuilding.