It's a good question and despite the easy breakability of melee classes, they have a lot of disadvantages as well. Because most of them have to get up close, it's easy for them to get separated, or worse, stranded, and hacked to bits, without chance for revival. So there's still some mental effort to do on their part.

I love magic, I think it's really powerful, I've just found MP management can be an issue. Other than that, I recall the spells stay pretty powerful and AoE makes them that many times more useful. B/C of JP buying and the way I personally grind, I can get second and third tier Fire/Ice/Bolt spells way earlier than I'm supposed to and just decimate the opposition.

Also, I would point out that one advantage a Priest has to a Chemist is that the Priest has tons of things to do (protect, shell, regen) on his/her turn when characters don't need medical attention. You could argue you could just wait them to get more turns but my preference is to buff. Also on my last playthrough I had a blast using the Oracle so they help with that too, as well as debuffing

Unfortunately, when it comes to the end of the game, to me it's all about spamming your overpowered moves before the enemy spams their overpowered moves and strategy goes out the window. In that case I don't recall what my party composition was but it was probably all melee with potentially a white mage/oracle. I've never used the calculator class because it seemed way too complicated for me but after all these years and a couple playthroughs i think i might be ready to give it a shot. I got a cool save on the PSOne Classic on the PSP right now, maybe I'll give it a whirl!