Chaosmancer
Legend
You dont need to have all the correct information. Diversify your spells memorized, and just roll with it. Now, I can still absolutely see your perspective, but its not the end of the world, never has been, to have to think ahead, and yeah maybe not all your spell slots are used in a day, or you picked some duds.
Sure, it isn't the end of the world. But... is it fun? Is it fun to think "Well, do I need to cast sleep once today or twice?" Because what ends up happening is people just default to the safest, blandest, most repetitive options. They won't pick a quirky spell, or a spell that may be useful in the right situation, they will pick the most guaranteed useful thing.
This already is happening in 5e. I've played a lot of clerics and unless I KNEW we were not going to be dealing with combat or that we would be dealing with an interrogation, I never prepared Zone of Truth. Which means that any time we are say, delving a dungeon and unexpectedly running into a situation where that spell would be useful... I never have it. Vancian casting is EVEN WORSE about this, because now I don't just need to decide to bring Zone of Truth, but I would need to guess how many times I'll find Guiding Bolt useful compared to Shield of Faith or Bless. If I do prepare Zone of Truth do I prepare one less casting of Pass without a Trace? Or do I drop a casting of Invisibility (Trickery Domain, btw)? What will I need? There is no way to know, so I might as well default to the most broadly useful spells the most times.