But I reject that Barnes will foreclose the Celtics from other opportunities the following season. Just as most every team will have cap room this summer, so will every team have cap room the following summer. Barnes would make about 85% of the 0-6 years max in 2017. Barring injury, he would be tradeable if a better free agent wanted to come to the C's. Furthermore, the last two CBA negotiations have included an amnesty provision. Even if Barnes were for some reason not tradeable, it is probable the C's could erase his contract that way if truly needed.
It's going to be 100% of his max. His suitors will include not only competitive teams with cap space, but rebuilding teams with cap space.
Yes, it's probable that if need be the Celtics could dump him. Then again, maybe not, if he continued to be average-ish even in a new environment with a featured role. Amnestying him after one year is extremely unlikely.
I'm ignoring the second part of your post other than to say it's unlikely that the C's will sign any of those players. Furthermore, virtually all of those players will be in the plateau-decline phase of their careers, which may make less sense for a team that is on the other side of the development curve.
But aside from that, I was to make it clear I was saying that Barnes' 2017 salary from a deal that he signs in July 2016, will be 80-85% of what the max will be for a free agent of Barnes' experience level who hits free agency in 2017, because of the cap increase and commensurate max salary increases.
The reason I point this out is because there will be teams out there who miss on free agents and will have large amounts of available cap space. Barring total regression by Barnes (which I admit is not impossible and why I do call him a risk, but so is every free agency signing to some degree), there will be teams who considered (and perhaps missed out on) Barnes in 2016, who have set aside at least his 2017 salary for a potential major free agent. I would be shocked if someone weren't willing to take him on. And again, in order for this to be relevant, the Celtics have to need his salary for another, even better, free agent.
I agree that amnestying him is extremely unlikely, due to the prior paragraph. But if Durant or Griffin or whomever wanted to sign here, and Barnes salary was in the way and untradeable, I am sure the amnesty provision would be used, should it exist in a similar form to the current CBA. Remember, the Celtics would not be on the hook for $60+ million of Barnes salary (even if it were wiped off the salary cap ledger). They'd owe $60 million minus whatever some other team was willing to bid. Presumably that number would be whatever some team was willing to pay him per year, if he were a free agent in 2017. Since we're assuming he's untradeable at $21 million, it would be less than that, but by how much? Would someone still be willing to give him $15 million per year? $17 million? $12 million? Regardless, it would shock me if the C's were forced to eat a majority of Barnes' contract.
My point is that I think the downside of Barnes is low. If he takes a major step forward, he'll be worth it. If he's just an upgrade on Jerebko, we'll be a better team next year and can still try again in 2017. 3-point shooting and the ability to defend the 3 and 4 is the type of player this team will be targeting.