For the next 5 years until the fashion trend wears off.

We used to write COBOL using thin client on mainframes. Then we moved to development locally.

The team here is part of the company that markets Codespaces. They're not actually representative of other companies, so I wouldn't take this as a direction all software development would be taking.

No. You can achieve the same benefits of local reproducibility with Nix/Guix. Guix is more composable IMO because you can program multi-service development environments entirely in scheme.