Melee combo is 12.7 seconds, not "6 casts at 2.5 seconds." Of those 12.7 seconds, 3-5.2 seconds are mandated to be in melee. If a mechanic forces you off the boss, that mobility is strictly locked off from you.
Swift and accel exist and are great sources of mobility but not to be used carelessly since each instant cast changes the rhythm of your casting. Yes even melee combo which is why you *have to* end off every melee combo with accel/swift or you'll end up misaligning your ogcds.
You can bank melee combos but you don't necessarily get more of them than you did before. Costs were halved but so was generation, with manafication being the only exception granting you a significant increase in mana generation. Even then, you also want to be careful about spending that melee combo carelessly because you want to end up at specific mana points to do triple combos under buffs and pot, something you don't really get to do if you're panic-meleeing just for the movement.
Even then, Red Mage needs to guarantee for those massive bursts that it will have 38.1 seconds of access to melee because it will be doing back to back to back melee combos.
The theme here is Red Mage's useful mobility has costs that have to be weighed. Is fleche far enough away that I can restore my rhythm and not drift it? Is contre? If I melee here will I have enough for burst later? Should I start the melee early since I'll need some mobility and I know I'll be forced out of melee?
It's not so cut and dry as "RDM only casts 25% of the time" as presented.
So let's say Red Mage can get 4 melee combos every two minutes. That's 50.2 seconds where instead of casting it needs melee access. Swiftcast and two accels adds that up to 57.7s. Assuming a gcd of 2.5, that leaves 24 casts, 12 of which will be dualcasted...with the issue of dualcast being that it's not bankable, so a lot of the time it will be wasted mobility. If we play the "don't use Swiftcast game" because even Red Mage prefers to swift raise over dualcast raise for a lot of reasons, that goes up to 14. That sounds pretty close to summoner anyway.
But 38.1s of the melee burst will all be in one go due to damage optimization, leaving 12.7 of that as free mobility, and again melee access is mandated to use this. Dualcast is useful when it's useful but it's very limited because so much of its mobility gets wasted by design.
Versus SMN who needs to make sure they're not trying to slipstream or ifrit cast at the wrong time. I agree with the argument that mobility tax is dumb, by the way, and believe that both smn and phys ranged should be brought up and ignore it. It's not like they're fighting any other role for a spot.