VS Code wouldn’t have won the mid-2010s editor wars if it was closed source (note that VS Code has not helped MS ramp people up to VS itself). The winner of that war was always going to be an open source editor, it was just Microsoft whose concept won out. Closed source editors like Coda failed to gain traction and even Sublime Text fell eventually.
If MS ever decided to discontinue VS Code or relicense it, there would be blood in the water. I guarantee you there would be multiple compelling competitors in under a year and probably a new open source winner with consolidation in 5.
So to answer your question: they would be forking Atom (which I think would’ve won otherwise).
Sublime Text fell because VS Code was just better, not because it was closed source. I switched from Sublime Text to VS Code, and didn't care one bit how open or close either was.
Not saying there aren't people who care, there are, but they are a small minority.
> So to answer your question: they would be forking Atom (which I think would’ve won otherwise).
Atom was far slower than VS Code, despite both of them being built on Electron. I wouldn't have used Atom, but I use VS Code.
It is entirely possible that some other closed-source editor with a superior package/extension system would have won, or the "war" would have been postponed until Rust was ready enough for Zed to come along.