To clarify, without weighing in on the merits of his complaint, what he is saying is that he wants to use Debian Stable's toolchain and is complaining that the ecosystem (crates on cargo.io) move to use the latest features so quickly, and don't have stable backport releases, which makes this difficult to do.
I think you're agreeing with me, but framing it as disagreement.
> the ecosystem (crates on cargo.io) move to use the latest features so quickly
They use the new features in new software -- implying he wants to use that new software. If he uses the old versions that predate the new features, this wouldn't be a problem for him.
Edit: Reworded to remove disagreement.