I think at this point monoculture is inevitable.

There are no new engines that aren't forks of existing engines aside from Servo which is still experimental.

Aside from EdgeHTML​ all the major browser engines are open source. It takes several years and lots of money to create a from scratch engine compatible with the majority of websites people visit.

There is no money made directly from the browser itself. All of the money is made from products or services around the browser. Which is not a viable model for anyone but the biggest companies.

Not enough people care about how unwise monocultures are. Network effects push people to the thing everyone else uses.

Rust has a monoculture. So firefox does/will have one

I don't think that Rust has been around long enough to have created a monoculture. :)

What is Rust's monoculture?

I think he means the rust compiler.

Ah, that'd make sense. We'll get there someday.