This is like the Ronaldo saga. We had the best attacker in the world (best GK in this case) and there wasn't anyone bar Messi that would've been an adequate replacement at the time. Bar Neuer, there isn't a GK that I think would be an adequate replacement for de Gea right now either but, like Messi, isn't obtainable.
The mistake that we shouldn't make that we did in 09 is settling when we should've gone for someone like Ribery. We shouldn't settle for a decent 'keeper like we did with Owen/Valencia/Obertan, we should go for the best we can even if they aren't on de Gea's level yet. All out for Donnarumma. We've seen how not having a top 'keeper after Schmeichel was a hindrance.