Would you really have been happy with a monoculture based around a very similar but slightly different and less mature language if it had gone the other way?
Why would there be a monoculture? JS would still exist for a good while after that, and if anything, it would've likely inspired more competition to flourish in the browser space.
Secondly, while the language may appear similar, a large number of its strengths come from actually breaking away from the JS ecosystem, and daring to take the 'less mature' approach. Without all that legacy, it turned out with significantly cleaner libraries and APIs, more intuitive semantics, and and an ecosystem+workflow is worlds apart from the JS style. The difference a few small changes can make in simplicity to lower cognitive overhead is astounding. Building a large, complex app in dart actually feels totally manageable! Yet, regardless of how many new JS tools and libraries come out, it always still feels completely overwhelming. Thankfully they decided to keep this part of dart in tact.
