It takes years for players to gain the level of experience annodomini - the problem isn't turned around that quickly. Even when youngsters reach the dizzying heights of World Cup potential, they also have to deal with the lack of belief in the team (and themselves) to do well. I can't remember exactly when the rule was introduced (not that many years ago, maybe 7 or 8 years ago) and existing club members would have had their contracts honoured which would also delay opening up places to Spanish youngsters. They then have to work their way up through the lower leagues until they are bought by clubs in the higher leagues - takes ages!
I always wondered why Spanish players were so much better than ours - then one day, I was watching a local team play near where we lived (in Spain) and they were learning to play on a pitch with not one blade of grass but hard, packed earth - that was levelled by dragging an old door across it lol, very impressive play nonetheless.