One of Discord's targets is web, and I don't think there is another good UI toolkit that could maintain that target with easy feature parity. Some UI toolkits like Godot (which is a game engine) are capable of targeting web using a canvas element, but that's probably not going to be as performant or as native feeling as a standard website. They could also use a separate code base, but then there would be large feature parity issues. They could also drop the web target, but that might make it harder for users who don't have permission to install software or use Chrome OS.

I feel like Discord is one of the examples where Electron makes sense. If only they allowed third party apps so desktop users who don't care about having every feature can have a better experience.