1) Originally the game had mana. Now it hasn't. Did you know that when you pledged it? In the fist alpha build there was no spell preparation. Now we are going to have it. That's the purpose of an Alpha: improving the game quality through iteration. The only null argument here is: "I like the game how it is (totally unbalanced, for the record), so nobody has to change it".
2) Again, don't take it personally, but: "You should impose yourself all the limits you want and leave other players free to enjoy the game as they want" is the wrongest idea I've ever heard about game design. Games DO have rules. Designers are paid to conceive these rules, and players are supposed to play according these rules. That's why we are having a debate about what rule would make this game better.
3) Being able to finish the game killing everybody is 100% about choices and consequences. Developers basically are saying: "you want to be a moron? Be my guest! We don't give you a game over. We give you the full consequences of your moronic actions".
This IS an old-school-RPG inspired by one of the best RPGs ever made.
This IS therefore a game with choices and consequences and complex systems.
If you want a consequence-free game, you've pledged the wrong game...
Pretty much this. It seems obvious that if a player finishes the game killing everybody, it'll leave the game world in a very different state than the one left by a player who finished it without killing anyone. I hope it will, anyway. I don't see how "being able to finish the game anyway, even if you killed everybody = no consequences".
It remains that we need more requirements and cost for powerful abilities. Cooldowns are a way to do that, but I personnaly think they're inelegant and I don't like them. Bringing back mana would already be better, for my tastes : "will I use those two powerful spells now, hoping this will end the encounter for me, or be more patient/tactical and use a greater variety of less costly, less powerful spells", is already a better choice to make imho. (granted mana/potions are a relatively scarce ressource).
I must also say that, being an avid p&p player (thus, I also like rules - disclaimer : I never played D&D), I like the idea of reagents, spell preparation time, etc. It's something that "feels" like a sorcerer "actually performing" magic, I think. It would make sense that highly powerful spells have that kind of requirement in a fantasy world.
To go further, I also like the idea of spells having direct consequences in the physical world, like, for example, a healing spell requiring to harm someone else. Magic invoking dark powers requiring sacrifices. Magic affecting the weather having a chance to go wrong and permanently freeze an area, etc. It would also make sense for a form of arguably nefarious and banned practice like source magic. (moar consequences : "am I gonna take the chance to use that very powerful fire spell to quickly dispatch those goblins, taking the risk to permanetly destroy every item to be found in this patch of forest ?") I guess It would be very hard to implement properly, though.
In the end, though, I mainly hope Larian will implement their vision on the game and not let themselves be influenced by design feedback from (potential) players too much. They're the pros, they know what they're doing (most of us probably don't) and they obviously have a pretty great idea of what to do with the game. (and I'm lovin' it

)