If you really feel the need to provide trade-offs for casting PrCs, the way to do that is the way the Archmage does it: giving up spell slots. That's absolutely fine, and in most cases it makes more sense than taking away levels of casting. Look at the Seeker of the Misty Isle again. Most of what it gets is already SLAs. 1/week discern location is in no possible sense worth a level of casting (as, even at 20th level, the level of casting gets a slot that could cast it each day and also a 9th level spell slot). But you could imagine someone who thought it was worth a 6th or 7th level spell slot for their character.
It's been stated, but the follow-up is pretty absent. Making Green Star Adept full casting doesn't make casters better. It makes Green Star Adepts better, but those guys suck and need the help. But the fundamental thing, the thing that is just completely ignored, is that this is not a power question. If you think casters need slower spell progression, fine. Make everyone use the Sorcerer progression. But the underlying logic is the same: having people do cool things is good.
Sure. There are a lot of spells. But that's really not the problem. The problem isn't that casters know too many spells. If that were true, the gap between Wizards and Sorcerers would be larger. The problem isn't that casters know too wide a variety of spells. If that were true, the gap between Sorcerers and Beguilers would be larger (frankly, the gap is probably in the opposite direction of what it needs to be for this argument to hold water, but that's a whole other thing). The problem isn't that that there are too many spells to choose from. If that were true, spellcasters would be fine in Core. The problem is that there are a limited number of specific spells that are overpowered. It's not a big list, and you can just fix them. Or even ban them. But, again, the problem is absolutely not the Wizard who casts fireball, or even stinking cloud.
Since you're making this claim too, I'll put it to you as well: what build does this. What is the build you can make with a partial casting PrC that is better than what you can do now?it will give casting characters more powerful and versatility