But fundeps
– because they don’t carry evidence, as they stand – feel less solidly rooted.
Can you elaborate? I don't understand where you are coming from yet.
(for the record: I'm not applying just “guarded by their own syntax” as a criterion, for me, this criterion is a sufficient condition for “no surprising new errors”. I also believe (believed?) that functional dependencies were quite standard and expected them to be here forever. I took TypeFamilyDependency to be a mild extension. I'm perfectly willing to be convinced otherwise.)