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