For me, that is *exactly* how it should be for casting a fireball, where making that judgment, in real time, of where the enemy and your allies are going to be when the spell goes off is part of the expectations of the combat system.
But the scenario of one person managing the fireball spell and being able to devote their entire attention to managing the fireball spell doesn't exist.
You aren't solely and completely focused on casting that fireball. You're a human being with one brain and one set of reflexes trying to control four characters at the same time while dealing with 5-8 characters controlled by the computer with inhuman reflexes and reaction time. You're targeting the fireball, and you're trying to counterspell the caster, and you're trying to backstab the enemy cleric and you're trying to heal the fighter who is trying to get in an enemy fighter's face and you're trying to move everyone at the same time to find cover from the enemy attacks.
[Since you appear to be trying to engage me in a sincere discussion, I will reciprocate.] I agree with your point that the player is going to be trying to manage not only the fireball-casting wizard but also at least three other characters all at the same time. And as such, this could be overwhelming and difficult for a person to manage. But here's the key point for me in all of this, including what I said earlier about the whole fireball casting business. You seem to be (I'm guessing; you can tell me yes or no) looking for everything working out optimally, perhaps even perfectly, across all your party members' actions. The fireball gets cast perfectly. The wall of force you wanted to place gets placed perfectly. Every one of your party members does their respective actions in the round perfectly and optimally. But I don't want this to be how combat works for me. I want combat to be sub-optimal and imperfect. I want there to be screw-ups and mess-ups and things not working out exactly as I had planned or hoped for. I believe this is how combat *should* be, especially in an RPG.
So maybe (Again, I don't know. I don't have data. I am making an educated guess ...) the difference between fans of TB combat and RTwP combat is that TB fans love making some sort of "perfect" plan for how combat is going to unfold across the whole encounter, or at least across each round in the encounter, and then they want to be able to carry out that plan in an optimal way (fireball gets cast perfectly to hit maximum number of enemies and no allies, for example), whereas RTwP fans don't care about having a perfect plan for winning the encounter and their fun comes precisely from winging it. I've heard very often TB fans describing RTwP combat as being "chaotic" or "confusing" or "messy." They are correct about this. The difference, I think, is that we RTwP fans like it and want it to be chaotic and confusing and messy, whereas TB fans don't.