If a browser engine continues to exist not on its merits but because its users are locked in, there is zero value in it. If 100% of people switch to chromium based browsers (an open source project) while they have free choice that's how it is. There's nothing inherently wrong with this.

We don't need browser engine DEI. Even the term monopoly is spurious in the world of open source software. Say if in 30 years we have 100% linux market share because open source won, do we need to protect Microsoft so they can lock people into Windows, like some sort of endangered animal program for proprietary software?

There's an inherent contradiction to apply the competitive logic of proprietary platforms to fundamental OSS infrastructure. They'll tend to be natural "monopolies" just by virtue of how resource intensive they are and the desire to standardize.