I don't have much experience with Planescape (I like Sigil, but I think the Great Wheel is a terrible cosmology), but does it have much problematic content that needs changing? Does it not fill its intended role as well as it could? Because those were the two main driving factors behind the changes to Spelljammer and Ravenloft.
Spelljammer replaced the Phlogiston with the Astral Sea because D&D's cosmology is already complicated enough, the Phlogiston isn't fun to play in, and "Astral" literally means "starry", so they just did the obvious thing and dropped the Phlogiston for the Astral Sea. WotC thought that the Phlogiston wasn't fulfilling the main goal of Spelljammer, having fun space adventures in D&D, and so they dumped the idea they thought was bad and replaced it with one they thought was better and cooler (being able to visit the corpses of dead gods in the plane of stars).
Ravenloft changed to get rid of most of the problematic stuff (changed Vistani into a multi-racial culture and got rid of some of the Romani stereotypes, dropped Caliban for Hexblood, changed up some of the Domains of Dread), but it also changed because the setting's main goal is to be D&D's horror setting. And the entire concept of "The Core" in Ravenloft doesn't work well with horror adventures, because horror adventures work better when the characters feel isolated and claustrophobic (stuck in a small space without anywhere to run).
I know that I think Planescape doesn't fill its role as well as it could, because I think the Great Wheel is bad, but I'm pretty sure WotC doesn't share that same opinion. They like the Great Wheel so much that they found a way to cram it into Eberron. So I doubt that WotC would "ruin" the setting in the way that I would. I'm about as hesitant about buying the Planescape book(s) as I am about Dragonlance, but for very different reasons.