tl;dr: A notable marketshare of multiple browser components and browsers must exist in order to properly ensure/maintain truly open web standards.
It is important that Firefox and its components like Gecko and Spidermonkey to exist as well as maintain a notable marketshare. Likewise, it is important for WebKit and its components to exist and maintain a notable marketshare. The same is true for any other browser/rendering/JavaScript engines.
While it is great that we have so many non-Google Chrome alternatives like Chromium, Edge, Vivaldi, etc., they all use the same or very similar engines. This means that they all display and interact with websites nearly identically.
When Google decides certain implementation/interpretation of web standards, formats, behavior, etc. should be included in Google Chrome (and consequently all Chromium based browsers), then the majority marketshare of web browsers will behave that way. If the Chrome/Chromium based browsers reaches a nearly unanimous browser marketshare, then Google can either ignore any/all open web standards, force their will in deciding/implementing new open web standards, or even become the defacto open web standard.
When any one entity has that much control over the open web standards, then the web standards are no longer truly "open" and in this case becomes "Google's web standards". In some (or maybe even many) cases, this may be fine. However, we saw with Internet Explorer in the past this is not something that the market should allow. We are seeing evidence that we shouldn't allow Google to have this much influence with things like the adoption of JPEG XL or implementation of FLoC.
With three or more browser engines, rendering engines, and browsers with notable marketshares, web developers are forced to develop in adherence to the accepted open web standards. With enough marketshare spread across those engines/browsers, the various engines/browsers are incentivized to maintain compatibility with open web standards. As long as the open web standards are designed and maintained without overt influence by a single or few entities and the open standards are actively used, then the best interest of the collective of all internet users is best served.
Otherwise, the best interest of a few entities (in this case Google) is best served.