That said, this is absolutely going to be changing now. We obviously can't keep relying on tarballs anymore. We'll find a new normal that will work for a very long time until some other critical issue arises and the cycle repeats.
For archival purposes, nothing prevents people from creating a tarball that contains the .git directory as well, which would preserve not only the current state of the project but its entire history.
And? You're speaking probabilities, not certainties. It's not relevant in terms of archiving. You don't guess, you don't hope, you simply 100% ensure that 10 days, 19 years, or a century from now you can build the same thing.
I agree that adding "extra stuff" to a tarball isn't a bad idea, and in fact, many already do!
My responses in these threads has been to the "why not build from git" logic championed here.
You want to build from a reliable, repeatable source. And as I mentioned, that can be git clone -> tarball, sure.