All the specifics aren't laid out here, but I am assuming Kyrie is signing a 5-yr max and AD would be appeased by Kyrie and AD here. I also assume that since KD wanted to come here that he would be willing to re-sign after his contract is up.
If these things are all true, then of course you do it. Next year may not be perfect, but you would still have Kyrie and AD. After that, you have a true superstar trio locked up for years. It's not like Boston isn't accustomed to paying out $30m+ to an injured player.