Gelo: it wouldn't matter.
If primary outcomes were very good, not to say excellent, a few things would happen.
Firstly, the importance of the grammar would start to evaporate. The vast and febrile competition to get into a selective school would start to disappear. If all primary school children carried their very good, not to say excellent education into secondary, then secondary outcomes would also improve enormously.
Secondly, you assume that all parents who enter this competition do so willingly and with enthusiasm. Not so. They'd probably rather not spend the money and time, and rather not put their child through the stress of it. And if they didn't have to, because they were getting a damn good education anyway, they wouldn't do it.