I was ambivalent about the trade initially but I think it would be a disaster for both the Cavs and the Celtics if it were voided now. Because it's made longer term intentions from both front offices public. From the Celtics' point of view they would have to deal with 2 players who were kind of the "heart and soul" of our chip-on-the-shoulder team of the last couple of years. It's a professional business but people have emotions. Some serious fence mending would need to happen. This isn't just NBA2k with players with ratings, locker room chemistry and attitude have a big influence on success.
Plus if IT is injured and decides to have surgery and sit the year out to make sure he is healthy for FA then we're stuck with Rozier/Smart for the year. It's possible that Hayward picks up the scoring slack, but I doubt we would be a better team than we were last year. And if IT isn't given the max and leaves next year then we get nothing for him.
As for the Cavs, they would have the same issue, dealing with a disgruntled player who might end up sitting out training camp to force a trade. They probably won't get anywhere near as sweet a deal elsewhere as the Celtics gave them - 3 players plus a lottery pick. They would have no leverage anywhere else.
It's in both teams' interest now for the trade to go ahead. What it will come down to is what price Cleveland will put on IT's possible downtime (if any), and what Danny is willing to give up (if any). At some point those two points have to merge, but there's going to be a lot of gamesmanship on both sides to get there, which could risk torpedoing the deal for both parties. In fact I'm nearly certain that this is just a tactic by Altman to try and scare the Celtics into giving up more assets to add to what was a great deal for him already.
A couple of things that should be non-negotiable though - giving up the Laker pick (unless it's traded for the Brooklyn pick, which won't happen) and giving up either of Tatum or Brown. Those should be off the table, even if we have to torpedo the deal. Because those things are our future.