I don’t understand why the control panel UI wasn’t modernized instead? Would that really be unfeasible? I think it still might have been less work than to maintain 2 coexistent “settings/control panel” apps and migrate from one to another. Sometimes you have to throw out the old code base and start from scratch. But if you do so shouldn’t you rather distrubute the result when your finished and not in a half-baked compromise-like state?
When excalidraw was mentioned in another comment I think it would also be worth to mention tldraw even though I don’t kniw whether it can be counted as an replacement since I never used draw.io.
https://github.com/tldraw/tldraw