Does Unity keep the editor closed source on purpose? Or is it currently due to legacy reasons?
Most of it has been marked internal and doesn’t have great support for overriding.
We can see from the asset store that developers are great at expanding Unity’s usability, and in turn popularity.
Imagine fully customisable editor themes, would be nice 
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I’m hoping when they convert everything to UI Elements it will leave it open to modification.
I mean most of the editor is still IMGUI, which is not easily (or not possible to) modified unless specific API’s are introduced for us to hook into. Making it public wouldn’t give us much room to modify things, and would just make it harder for Unity to change things without creating lots of breaking changes (while their current ethos is to create as few as possible).
Though newer systems like Overlays and Tools are public, UI Tooklit API’s for us to hook into. So I’m hoping as more things move over to UI Toolkit, we do get better ways to interface with them.
It’s still a bit silly that windows like the Hierarchy, Inspector, and Project Window are still internal. We should’ve been given at least some high-level way of interfacing with those windows long ago.