Why not make it so you CAN cast while running, but have it inflict a slight slowcast effect? So that fire spell you want to cast that normally takes 2 seconds takes 3, that flare spell that normally takes like 5 seconds takes 7, and so on. That would make it so mages have the option of casting while running, but the trade off of mobility is a delay to spellcasting.

Would make sense, as it's harder to chant and wave about while running... but not impossible. And it would encourage casting while staying in place while not outright banning casting while running.

Could even make it a toggle option probably, where those who want to use movement to cancel a spell can have it so it does so, and those who prefer casting while running could have a hotkey assigned (preferably escape key, but anything really) to cancel a spell instead.