I don't have any insider knowledge. It makes sense to me that a trillion-dollar platform company will at some point want to control their browser destiny, and not rely on their competitor's roadmap.
You may very well be right. I have no special knowledge of internal workings at MS. Having said that, I can guarantee that there are factions within MS that were against getting rid of Edge and factions that are all about MS building their own browser. It's very possible these factions may win out at some point in the future. They certainly have the talent and resources to work on Chromium/Edge independently.
The benefits to common engine are too high.