Thanks for all the helpful comments and feedback,
though more is still welcome of course.
So I am leaning now towards not installing fedora-repos-modular by default
(rather than disabling the modular repos by default): also for upgrade compatibility.

I have started a Change proposal draft, which I think should be ready soon,
with current working title "Turn off modular dnf repos by default".
Perhaps "No default fedora-repos-modular" would be more precise.
Or any better suggestions (that don't sound like modularity is being removed)?

I think the actual work (PR's) required is not huge,
but if someone is particularly keen to join the effort let me know.

Jens