There is downside for the Celtics in terms of Crowder and Thomas returning with an attitude if the trade falls apart, but I don't see that as worse than the downside for the Cavs. Cleveland would have to give up Crowder, give up the Nets pick, and deal with Irving who definitely doesn't want to be there.
As someone who wasn't a fan of the trade I don't care if it falls apart, and I really don't want Ainge to give up anything else. If the deal was contingent on the Cavs doctors thinking Thomas would be fine by XXX date and they don't think he will be, oh well, so sad, too bad, move on to Plan G.