I would've so loved to see that crossover! I'm sure it would've been a blast.

Yeah, they're both extremely busy and adding to that the way censorship has been clutching their pearls about dramas based on BL, it's definitely better to not feed the CP fans for now. (Pretty sure that's why XZ has been doing all these modern het dramas that also are all "China is so awesome, I love my country"...)
I do hope that there's at least a possibility for a cast reunion in the future, when enough time has passed. Though I don't know if that is sth that's done a lot in c-ent, but it would be nice.