It would be a deviation from existing practice for a collection ABC to
exist in typing but not in collections.abc, and it would mean the
inheritance tree for the types in typing.py would be different from
that in collections.abc.

These ABCs, while not so useful by themselves, are building blocks for
the more useful ABCs. They also give people the right terminology.
That arguably is the biggest problem here -- it's clear that people
desperately want to be able to talk about the common API for sets and
sequences, which happens to be __len__, __iter__ and __contains__. But
it's also clear that people often believe that that common API is
called Iterable, which it isn't.