I have a question for you. What do you think is easier and more efficient for them? Reducing a single bosses HP? Or increasing the potencies of several abilities for several jobs and then having to do a bunch of testing to make sure they're not over or under tuned?
Square Enix is a business and it's going to prioritize efficiency. Square obviously feels that every job is situated relatively well or else they would have gotten buffs outside of paladin and warrior.
They're clearly comfortable in their belief that all jobs can do all content.