A lot of downsides to Cleveland rescinding the deal, but one underrated one - Kyrie waived his 15% trade kicker to come here. It's unlikely he'd do the same to go to a less desirable location. That makes him less appealing to his destination but also means Cleveland would have to take more salary back, which has a multiplier effect with them being well into the luxury tax, and the repeater tax to boot.
They can find another taker for Kyrie, but it's dubious if they can find a comparable offer. And reversing the trade doesn't just throw their rebuild into doubt, it will cost them potentially tens of millions of dollars. They don't have a ton of leverage here.