The volume of geostationary orbits is theoretically zero, it's a 2D circle. It's true that most applications can work within some tolerance around that mathematically perfect orbit, and it's also true that volume of all orbits at all inclinations near geostationary altitude is huge. I'll grant those caveats, but I don't think it's at all sensible to waste the best parts of those orbits, especially given the deorbit timelines. It's one thing to gamble with Kessler syndrome that will last a couple decades and cut off our generation from LEO, another if it lasts for millennia and cuts off our civilization from GEO.
There was an episode[0] on French radio show "La méthode scientifique" in October 2019 about these questions (following a near-collision between SpaceX and ESA satellites where apparently SpaceX just played dead and ESA people had to maneuver).
They talked about a lot of things, but IIRC GEO issues were very low on their priority list. This is a very thin doughnut but still a very very large on considered scales.
