Microsoft just did whatever they wanted with the web "platform", and so will Google.
In Microsoft's case what they wanted was nothing. They weren't a web business, saw it as a threat to their platform leverage, and so just left it abandoned and stagnant for years.
Google is simultaneously better and worse: they won't leave it stagnant because the web is their platform, but on the other hand they have a lot more to gain by abusing control of it.
We already have a number of Chromium based browsers that go against some of Google's most fundamental interests (e.g Brave).
Costs matter, and Web development costs are high. Google benefits from coordination, funding, and one migh presume, cost advantages, which would be exceedingly difficult for any comparable US or EU effort to match.
Development in lower-cost-of-living regions, perhaps most viably China, might pose an alternative.
Open source is that option. The economics of starting from scratch vs starting from Chromium's latest commit are fundamentally different.
I'm not saying that it's easy, only that it is not remotely comparable to the IE situation.
MSIE was bypassed not by a code fork of MSIE (itself originally based on the Spyglass browser, which was a fork of the NSCA's Mosaic codebase), but by independent implementations of an HTML-standard parser. Microsoft had some influence over Web development (noteably through ActiveX) but far less than Google has now.
My point is that Open Source of itself is not sufficient, and moreover simply is not viable. Glibly asserting that it is ... is utterly unrealistic.
Though the alternative of forking a Web-like markup and transport, as Gemini is attempting to do, is one option. For other technologies which have become sufficiently baroque, similar worse-is-better alternate paths have been pursued.
Otherwise, this is an antitrust issue, and Google very badly need busting.