I think many of you are looking at this the wrong way. It depends on two things: 1) how well IT plays the next two years, and 2) how well the team does. If IT is a top 3 player on a team that wins 55-60 games in 2017 and/or 2018, and at least gets a couple of wins in the conference finals, you, to use his words, "back up the Brinks truck." Even if IT is the best player on a team that wins 45-50, and makes it to the second round only once, you probably mutually move on, because you need to go a different direction. Or, if IT really is only a bench player on a top team, then $20 million a year is too much, but $15ish million would be fine.
I happen to think the Celtics will be a top 5 team and that IT will be one of the 3 best players, so I think you pay him. But time will tell.