We very rarely, if ever, sign the finished article. We buy players with the intention of developing and improving them, we do not sign the very best players or what could be defined as players hyped in the media because they don't represent value for money.
Even Fernandinho (the oldest outfield signing in recent years at 28) was bought with the intention of turning him from a well respected attacking midfielder into the best defensive midfielder we've ever seen, certainly in England, in my opinion.
It doesn't always work out but that's the intention. To expect us after all the success you've mentioned to change that transfer philosophy is wishful thinking. You might get the very odd exception (e.g. Koulibaly) but I personally don't think even that will happen unless Napoli significantly reduce the current asking price and he represents better value for money.