You are kind of making my point for me. You are saying that you only want to cast fireball. As I previously stated cooldowns make you rely on a more broader set of tactics, than just casting the "best spell" of that level every time, only deviating when you have a fight that has resistance to that spell. It also doesn't become OP, if you know that every encounter everyone will have access to all their abilities everytime, you can ramp up encounters to be more challenging, and take out small encounters that would be utterly meaningless. Also the other side gets the same access that you do. Nobody twiddles their thumbs in other games with cooldowns, waiting for the right one to come off CD, this is not a good faith argument at all.
One of the things as a DM/GM you always have to consider when making encounters is how difficult to make them. if you just make them all the same challenge rating, when they have to do multiple fights back to back in a dungeon, your party will either run thru it, or die horribly. Setting up pace and balance of encounters is a huge timesink, and in PnP you can at least make adjustments on the fly.
Also if you really need to cast that fireball a 2nd time, it will come off of cooldown in time for you to use it again, or you might have to adapt to the battle, because the enemies aren't grouped anymore.
I think you are so convinced cooldowns are the best you are refusing to see the problem. If you use cooldowns, it makes martial classes obsolete. They are already weaker as you increase levels but once you allow full casters to cast their most powerful spells every encounter, it's over. Why bring a fighter when a caster can do more to more mobs every time? My example only included fireball as an example. Your entire argument is about fireball alone. That wasn't the purpose of my argument. There's plenty of other wizard spells as you progress that completely outclasses what a martial class can bring if they can use it in every encounter. That's why they are balanced so they can't be used all the time.
Dude, people wait for cooldowns to recharge all the time. If the spell is powerful enough, people will wait. Play MMOs and you'll see.
So what you are saying is, you want Diablo-esque gameplay where you blow up large groups of mobs. Drop a fireball, then lightning bolt. While you are at it, once you reach level 7, toss in an ice storm. Wait for recharge and repeat to the next encounter. How very tactical.
If you are using 5e rules, you can't use cooldowns without compromising the balance. Otherwise, it's a totally different system.
