That's another fictional aspect. That a distro will simply subsume a random third party upstream for one non-gnu package (or 5 or 10), and change the whole distro model and go proprietary.

I will let you in on a secret, the "stable" distro model itself is largely a lie. So called "stable" distros, even well funded ones, can barely do the minimal in that aspect. The only exception is maybe Red Hat because they employ people who do a lot of upstream development. But even in that case, that only covers a small fraction of what they package.

Distros need good upstreams to avoid responsibility, especially when it comes to security updates, not because they want to subsume all of that responsibility at some unspecified point for some unspecified reason.

The fact that this gets brought up whenever one more non-gnu-licensed rust package (or 3 or 5) is getting adopted, when non-rust literal thousands are already there, including many core dependencies, is what gives this FUD-like argumentation disingenuous vibes (assuming originality and non-ignorance).

Even arguing that "it's a clear pattern" wouldn't work, as that also wouldn't survive fact-checking scrutiny. For example, Ubuntu switched from the multi-licensed systemd to the GPL-only chrony for NTP purposes not that long ago. Where was that supposed "pattern" then?!

EDIT: btw, all "non GNU" mentions in my original comment are about the license. All use non copyleft ones (with the exception of MPL for a couple of packages).