Honestly, if the battle isn't over by the time your mages are out of MP, you probably were screwed either way lol~ but recovering MP is really not a very difficult thing to do in that game anyway. There are a ton of ways to do this.
There are also several workarounds to protect casters, like with mantles, reaction abilities... etc etc. So many possibilities to cover their weaknesses (or just use Blade Grasp and be OP, lol~ which ironically enough can only reliably be pierced by magic/special abilities).
You also say magic has limited range, but don't forget a lot of the melee type classes have only 3 move. They're probably not gonna reach your mages unless they got some move+ gear / abilities anyway. This also isn't taking into account height differences and what not, where mages also have the advantage. Regardless, you wouldn't be casting with your mage completely unassisted anyway / not checking turn list or move of your enemies. It's very easy to to get your mage killed if you don't check these things.
Then there's also things like how with "tracking targeting", you could actually target one of your party members with an offense spell, and have them run into the enemies with it. This works particularly well with Summoning Magic, because of it's innate trait of only hitting only your enemies if it's an attack type, and alternatively only hitting allies if it's a support type. Or hell, maybe you could use a character with ele-absorb equipment and simultaneously damage the enemy whilst healing the targeted character.
Hell, if you're so worried about MP and cast times, just give your mage Draw Out/Iaido. Insta neutral-element Summoner-esque abilities based on magic attack that ignore faith, no charge time, comes with some nifty support abilities like AoE heal, Protect/Shell (Haste/Regen if you're really far into the game) at 100% chance, AND comes with a uber long ranged Earth Slash type thing later on.
There are many things you can do, really.
And while I do agree MNK has awesome skills, don't forget that all their support abilities have 0 vertical reach. Yeah... having a character die on a single tile by themselves with even a 0.5 height dif for Revive/Chakra/Purification sorta sucks, haha. It's not -totally- foolproof. Really amazing, but not foolproof. Also the problem with Revive/Phoenix Downs is how they only revive the character with single digit/critical HP, whereas Arise brings them back with full HP, which to me feels a heck of a lot safer if they're dead near enemies.
Also I find unless you start using Elixirs, potions do not scale very well once you start getting remotely to the higher end of levels. At least Cure/Support magic are significantly buffed with Zodiac compatibility + Faith spell + Magic stat, which makes them a lot less cruddy and have room to grow beyond rather than stay static numbers like potion items.
And hell yes you better be abusing the Zodiac compatibility system. The first thing I do at the beginning of the game is get 4 units that are all "Good" compatibility with each other and Ramza (obviously). Makes a world of difference for magic.
Also after the first time I beat FFT, I never actually used the unique characters (aside from Ramza, obviously) and purely stuck with generics in subsequent playthroughs. I just found it more fun that way.
I really could go on and on so I'm gonna stop myself, haha.