I can't even remember reading any reports of read only failure due to flash exhaustion. I do remember boat loads of early SSDs, mostly sandforce based ones, spontaneously dying due to firmware bugs.

Electronic parts can fail silently and without warning in either case, e.g. due to a tin whisker producing a short somewhere.

Mechanical parts being less reliable than electronic parts in HDDs, and their relatively slow degradation, allow for an earlier warning when data can still be recovered but the drive should rather be replaced.

HDDs also have controller firmware and it's not unseen that they kill hard drives early.