In e.g. English, why do we say that better and best are inflections of "good" and not derivations of "good"?
Why is tastiest commonly understood as an inflection and not a derivation of tasty?
I think these words have meanings as different as some words understood to be derivations, like lord and overlord, or angel and archangel.
It's not possible to generate overlord and archangel based on common predictable rules, and -er and -est for degree of comparison are much more productive, but there are some fairly regular derivational affixes too.
Are there any languages where it's easier to argue that degree of comparison is actually expressed via derivation and not inflection?