The biggest danger to the web in terms of control right now is browser monopoly, not engine monopoly. And this move is probably the most effective way at combatting that. I think this is the right move at this time not just for Microsoft, but for the web.

Borrowing from a previous comment I made, think of it this way: do you think it helps or hurts Google to have every version of Windows come pre-installed with what is essentially already Chrome, except, of course, it will probably have Bing as its default search engine. Do you think the odds of people just using Edge to download Chrome and nothing else go up or down with this move? Do you think it helps or hurts Google to have most tech people not bother telling their parents to download Chrome anymore? There is significantly less control from "owning" an engine than owning an actual browser. I don't think I would have had much of an issue with the dominance of IE 20 years ago if I knew I could compile and modify (and release!) IE myself.

If you care about the state of search monopoly, out of control ads, and identity on the web, then you should be happy with this move. This is more akin to most browsers now having a common starting point. The problem with browsers is that if you truly want to make a new one you need to somehow replicate the decades of work put into the existing ones. What that means is that before you can exercise any of your noble privacy/security/UI/whatever goals, you must first make sure you pass Acid 1 and replicate quirks mode float behavior and etc. etc. etc. This is a non-starter. But now, Microsoft can launch from Chromium's current position and have a browser that can actually compete with Chrome. It's as if they've taken "engine correctness" off the table, and can compete on cool features or "we won't track you" or anything else. Websites will work in Edge by default, so if you like that one new feature in Edge, you can feel OK switching to it without compromising devtools/rendering/speed/etc.

Now I know that the initial response to this is "but Google will call the shots!". Not if the way this has gone down every other time has anything to do with it. Google's Chromium started as KHTML. When Apple based WebKit off of KHTML, the KHTML team had very little say in anything and they eventually forked of course. Then Google based Chromium off of Apple's WebKit, and once again, there was very little "control" Apple could exercise here. Sure, they remained one monolithic project for a while (despite having different JS engines which just goes to show that even without forking you can still have differentiation), but inevitably, Chromium was also forked from WebKit into Blink.

And there should be no reason to think the same won't happen here, and it's a good thing! Microsoft in the past couple of years has demonstrated amazing OS culture. I can't wait to see what the same company that gave us VSCode is able to build on top of Blink, and eventually separate from Blink. Ironically enough, the worst thing that could have happened to Google's search dominance is have Blink win the "browser engine wars": we all agree Blink is the way to go now, so we can all start shipping browsers that at minimum are just as good, and won't auto-log you in, or have their engine set to default, or etc. etc. etc.