Cooldowns also add strategy, having to think about when to use them, and cooldowns are also the core mechanic most other rpgs use, some use mana, some combine both.
I don't know if you are familiar with 5e but I think cooldowns would be a mistake. I think one of the biggest mistakes Larian made in EA is not including level 5. It's a major bump in power for the characters and people who are unfamiliar do not understand what lies ahead. Martial classes get their second attack. Casters get 3rd level spells. It's the reason why levels 5-10 are considered the sweet spot of D&D PnP. You have just enough powers to make things interesting and you aren't bogged down with too many powers that drag out combat.
I'm going to use everyone's favorite spell: Fireball. It's a 3rd level spell that does 8D8 AoE fire damage spell. It's auto hit and if you make a DEX save you take half damage. Notice a fighter would have to use action surge and hit with all four of his attacks to match the damage of one fireball. And that's just one monster. Fireball is AoE. Clearly, the spell is significantly powerful. Wizards can cast it twice per long rest.
Now, if the resting mechanic is used correctly, wizards shouldn't be able to spam fireball. They would have to conserve it for either large groups of minions or a boss. Larian NEEDS to fix the resting mechanic or yes, like you said, people will just long rest before every encounter. Have four wizards and drop eight fireballs. There's nothing that would be able to absorb that at level 5.
Let's go over cooldowns then. How would you implement fireball cooldowns? Every encounter? That's OP as you can see. A longer timer? Well that means players will just stand around twiddling their thumbs until it recharges which is equally pointless as the current resting system. Nerf it so it becomes a glorified fire bolt? I guess but then it takes away from a hallmark wizard spell. As you can see, just throwing out cooldowns in 5e presents lots of new issues. And this is just one spell.
And I disagree that cooldowns are more tactical. It's less. In 5e, the wizard has the choice to cast fireball twice in one encounter if the party feels they need it. Sure, the wizard can't cast it anymore until a long rest but the choice is there. And in a desperate situation, it may be deemed necessary. Cooldowns? Nope. Not an option.
D&D isn't about spamming powers. It's about resource management. As players you had to make calculated risks on when to use your most powerful weapons at the right time. Old school, if you failed to do that, you'd die. But D&D has been watered down so much it's actually hard to have a character killed. I never felt concerned for my characters playing BG3. Why? Because the game mechanics are a joke. There's no threat. No sense of adventure. It's just a bunch of OP characters plowing through the game. It lacks depth of tactics. Perhaps that's why games like Darkest Dungeon and Dark Souls are popular now. They actually offer challenge to the player. But that's another topic.
