We can absolutely renegotiate-and-extend IT. He'll only be eligible for the six-year max, $25.5 million in year 1, which would help maintain long-term flexibility. We would absolutely be able to keep KO in this situation too -- his cap hold fits in fine with the IT extension.
It's also not impossible for us to renegotiate and extend Avery, depending on a couple of variables (the draft lottery, if Yab stays abroad one more year). Say we're able to give AB a $6 million raise next season and sign him to a 5-year deal. He'd have to get more than 4 years, $77 million in free agency the following year to make turning down such an extension worth it. That's probably towards the top end of his market value, and he'd likely accept. This would probably let us keep all three of IT, Smart, and Bradley going forward, as IT and AB would cost about $42 million in 2018 instead of $50 million. At larger salaries, they'd also be able to bring back more salary on their own in trades, should a younger players begin to usurp a role in the future.
We could only bring back Amir at the room exception. Ideally we'd actually use it this year, whether or not Amir is willing to take it.
But the team would look largely the same next year, with another top draft pick and Zizic replacing guys like Zeller, Jerebko, and Amir. I guess Nader gets to come along for the ride too instead of Mickey, and there'd be 1-2 Gerald Greens on the roster. It's probably a better team than this year, as no major contributors are being lost, and continued growth in players like Smart and Brown create internal improvement.
And it's sustainable, because the next year yet another top pick is added, along with likely Yab and another 1st, and the only guys off the roster are at the true fringes -- min salary players from the prior year. Depending on how much Olynyk and Smart ultimately get, the team might be in the luxury tax, but only slightly so. The following year the story is similar, although probably without quite as high a pick, but still potentially 3 1st round picks, again replacing some departed players. The upside is there for the team, because if Brown becomes that super star, or another draftee does, it will be added to a base 50+ win team. The current core of this team could remain here for about another 5 years, as the new core is ready to take the baton from them.
Meanwhile, Ainge will still have good salaries to trade whenever an opportunity worth taking arises.
The chances are very very low that Avery signs for that kind of money. If you browse the 2016 free agent signings, its pretty obvious that paying him $20M/yr would be getting him for a serious home town discount. He's not Evan Turner or Evan Fournier. As long as he doesn't have a major injury before then, he's much closer to Nic Batum or Bradley Beal in market value.
That being said, Ainge will make a trade if he thinks he can get more value back than he is giving up. Otherwise, I'm pretty sure they will take the financial pounding for a year or two, and find out what the youngest assets really are.
If we do nothing and play the string out, the smart play is to trade Bradley or IT on new contracts in 2-3 years. Who we keep will depend on how good the Brooklyn picks turn out. If Fultz or Ball is a budding superstar, we'll know by the end of their second year, and it will probably be IT that has to go. If they are just really good prospects, we'll probably have to trade Bradley. We may even trade both. But I don't think we aren't going to waste either guy to save some money for one or two years.