It probably is a limitation in the current game engine, which is one of the reasons why I'm interested to know if they have any plans to improve on the existing game engine for the long term, now that we're ten years out. They pulled off a much more technically difficult feat in rebuilding ARR while 1.x was still running.

If there is a technical/financial limitation that definitively prevents this, then there is an alternative that might improve things. All you need to do is to create an untargetable, invulnerable mob that executes actions on behalf of the boss, from outside the arena. Casts still happen, but the boss no longer needs to pause movement to execute them because another mob is performing them in their stead.