I don't know about their financial situation and their objectives but as far as I know, they're building team around Leao and Theo (and some other promising additions). They didn't sell him once he was going through a fire form, so I don't think they will now. Indeed he's kind of inconsistent, but honestly he's THAT good, that even while he's underperforming, he tends to be impactful and productive in term of assisting at least.
Leao to Chiesa is a massive downgrade. The latter has to make a step back to review his career IMO, or perhaps be patient and see who's coming to replace Allegri, so maybe under a new regime, he could pick his form. But even at his best, Chiesa is still inferior to Leao, in every single aspect of the game.
Chiesa was never THAT good. He is known for his tenacity and great stamina. So, I wouldn't say only concern regarding Chiesa is being injury prone. I mean, Dybala is injury prone as well, but he's still on another level once he steps his foot on the pitch.