I'm sorry but you clearly don't know what you're talking about.
1. Universities do take GCSEs into consideration (or at least the half decent ones do)
2. Employers don't care if you got an A* or C? Don't make me laugh. There is a huge difference between an A* and a C, and they know that. Imagine you're sitting there with 2 CVs. One is full of A*s and As, the other Cs. Which candidate one do you employ? I know which one I would.
3. Everything you do after GCSEs are more important, you're right. But what if you don't get the opportunities to do what you want, like a good career or a degree, because you messed around at school?