alt Hacker News"Source Available" means that it can become "Source Unavailable" overnight.
See the "Our Machinery" fiasco.
Yes, Open Source isn't a complete defense against this (especially when there are copyright assignments). However, it sure makes it both a lot harder to pull off and a lot less useful to even try.
"Open Source" can also become "Source Available" overnight. See Redis, Terraform, etc. In the same vein, "Open Source" can also become "Closed Source" overnight.
In neither case does the change apply retroactively. It only applies to new contributions after the license change.