I was at Microsoft (and on the IE/Edge team) when that movement started. The writing was on the wall; too many developers were targeting (and testing in) Chromium (and/or Webkit) alone. The iPhone in 2007 pretty much claimed the mobile-web as its own.

Massive efforts were made to bring IE up-to-speed. Support for -webkit- prefixes and more were added, but that proved to be too little, too late. IE 11 eventually even had pretty stellar ES6 support.

Forking Trident into EdgeHTML (for project Spartan) was a good move, but still not the right move. Hundreds of thousands of lines of code later, it was clear that Edge would need to adapt or die. I left before the decision was made to drop EdgeHTML (I went to Brave), but I think that was the right decision for Microsoft.

I've seen saying for years that I think Firefox will eventually do the same, or suffer the same drift into irrelevancy that Microsoft had been experiencing for so long.