"Aren't any good for Cube" seems like a bit of a stretch to me. The "front halves" on a lot of these are good enough for players to want to run, even if they aren't always casting the prepared spell side. Emeratus of Conflict is actually a good example of this; a 2/2 First Strike forI'll be honest, I don't think any of the prepared creatures are any good for cube. Like, Limited must be a ton of fun, it's a shame I cannot play it anyhow. But I don't seem them making them into most cubes.
The bigger issue I see with the mechanic is that it's yet another complicated "backbone mechanic" with only a few cards existing at any one power level. The power band for Prepared cards is too wide to get more than a handful into a Cube of any given power level, meaning that this high-complexity mechanic is going to be practically impossible to get en masse without essentially building a Magic: The Gathering: Secrets of Strixhaven (TM) set Cube. As much as I think getting rid of blocks has been a net positive for set design, I think mechanics like prepared would be better served by appearing in multiple sets throughout a given Magic year.