[Armin Rigo]
On another level, would there be interest here for me to revive my old attempt at throwing away this messy procedure, which only made sense in a world where reference cycles couldn't be broken?
Definitely.
Nowadays the fact that global variables suddenly become None when the interpreter shuts down is a known recipe for getting obscure messages from still-running threads, for example.
This has one consequence that I can think about: if we consider a CPython in which the cycle GC has been disabled by the user, then many __del__'s would not be called any more at interpreter shutdown. Do we care?
I don't believe this is a potential issue in CPython. The user-exposed gc.enable() / gc.disable() only affect "automatic" cyclic gc -- they flip a flag that has no bearing on whether an /explicit/ call to gc.collect() will try to collect trash (it will, unless a collection is already in progress, & regardless of the state of the "enabled" flag). Py_Finalize() calls the C spelling of gc.collect() (PyGC_Collect), and I don't believe that can be user-disabled.