It would remove most of their rotation and change it to Verthunder/Veraero.
That'd be an awesome cooldown to have, for a short window, but it'd be awful as a core rotation.
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Different casters have different cast times and multiple casters have instant casts.
For an example of both, BLM both has a few casts that are longer than the GCD (Fire aspected Flare is the main one that comes to mind) but it also has the ability to instant cast any spell (such as fire aspected flare). So you need both cast time and GCD.
A few casters have some casts that are actually faster than the cast timer, like Astro's attack spell. And you don't want to get rid of the GCD for them because that small period can be used for movement, since optimally you want to be casting GCD after GCD, so without that small period, they'd have less time to move, an issue WHM has.
I think that another part of the reason Casters have the charge times is to, well, make them feel like spellcasters. Ranged and Melee DPS classes don't have cast times, at least now they don't *glares at HW Wanderer's Minuet*. When you play a Spellcaster, you want to feel that glorious rush of power as a spell goes off at just the right moment, or the terror of an enemy approaching while you desperately make your incantations and such. Having casting bars helps bring that feel into Caster classes and helps their identity compared to other roles.