You're correct. The Tome of Magic actually warns against trying to create new vestiges and instead suggests re-flavoring existing vestiges for balance concerns. I believe it is possible to create additional balanced vestiges, but that it would indeed be very difficult to make them good. Making them around spell-casting seems even more daunting a task. :smalleek:
As to the slot vs PP system concerns; frankly I don't think offering options is ever a bad design, unless the options are horribly unbalanced. For the same reasons as listed before, the Psionics system is in fact a beautiful gem. It would loose all of its mechanical value if it used the same x/day system that core magic uses. Some players don't like x/day magic, but can play a caster with psionics and be happy. That is good design. This is one of the major problems with 4th edition for a lot of people - the lack of variety. All the classes use the same mechanics, so if you don't like those mechanics, then you're out of luck. It's not that the 4E system is a bad system, but if you don't like the at-will/encounter/daily power system, then there's not much for you.
I've heard that the 4E psionics is making it a bit more interesting by giving more at-wills that you can spend points to improve like Encounter/Daily powers; and that sounds pretty cool. I don't know much about it though, but at least it offers some different options.
That's not a design flaw. That is a design feature. That is in fact a measure of good design. Far from the opposite. You're providing an alternative way to do something out of the box in a way some people prefer. If the party wants a nice sorcerer like character but someone dislikes playing a sorcerer, they might prefer playing a Psion. Since it's balanced, there's not a problem with this. Some people prefer to play intricate multi-class combinations of warrior/mage/divine classes, some people prefer Bards.
You could remove spell components and convert it all to XP costs; remove somatic and vocal components from spells; convert existing magic into a pool-based system instead of x/day to make it more attractive to people who prefer such a system; remove arcane-spell failure; and then make sure that the system is balanced; while leaving people who like those things and the x/day system with nothing.
OR you could just make the EPH/SRD psionics available, and be able to cater to both types of players without a lot of terrible imbalances, an even wider range of character options, and without all the legwork.
Yes, I still don't believe that even the plan for the EPH was a bad design decision. In fact, it seems like a truly brilliant one.