We are causing a major extinction event that is up there with the worst ones the Earth has experienced. That said, the end result is likely to be that we either go extinct or come very close and the remaining life on Earth picks up the pieces and starts re-evolving new biodiversity from the diversity bottleneck.
It's happened before and life on Earth, as whole, will come out the other side of this bottleneck and move on (unless we've tripped some unimaginable feedback loop).
...it's just likely we won't be included in that life.
...and a major extinction event on this scale is nothing to sneeze at either. Really it's chilling enough that we, as a species, are potentially having a similar impact on Earth's biodiversity as the asteroid that killed the dinosaurs. There's really no need to exaggerate that.
If I were Musk I would be building these now.
Even a completely 'trashed' earth that renders billions displaced or suffering amidst ecological catastrophe is still infinitely more hospitable to life than celestial bodies that are completely devoid of that benefit in the first place.
https://www.youtube.com/watch?v=g1Sq1Nr58hM
But more pragmatically, neither Biosphere project was considered successful enough to make this workable on earth at any large, or even small, scale. There is still much more work and research to do here. Any attempt here currently has a hard dependency on the larger ecosystem to supply it with inputs.
Musk working on building moonbases on earth looks a lot like what doomday preppers are already focused on.
Why should descriptions 'help'?
> to which there is no known better alternative
Why should there be? The only reason I can think of is if you have a prior religious/metaphysical belief in a specific species (in this case H. Sapiens) having a 'destiny'. If OTOH you see species as contingently arising, well, then: they come, they change, and sometimes they overrun, and go.