Originally Posted by Hanuman
As such, most PrC's CAN be replicated with base classes, and usually are.
Just because you CAN do something with the existing rules does not mean you HAVE to do it that way. If the designer of the class wants nonmagical robots, so be it. If I want to make a custom monster specifically to be perfectly in line with something from fiction rather than a close approximation, that is also fine.
You can make a holy warrior by multiclassing cleric and fighter, and yet there's the paladin. You can get a pretty close approximation of a samurai with just the fighter, and yet there's not one, but two samurai classes. There's nothing wrong with not wanting to approximate.
Telling the designer that he/she should use magic/magical constructs/magical power sources is not at all useful when the design goal seems to be to achieve similar effects specifically without magic.
Have you checked out d20 Future's robot rules
? They might help you modularize your robots to make them more customizable.