Just because they are key players in the team doesn't provide evidence of how big a signing they were originally. Robinho was our biggest (in terms of PR) signing at that time, and he didn't work out.
The fact remains, Yaya and Silva are both bigger names in the footballing world since joining City. Yaya was a bit of a coup (although many questioned if he was an overpriced Barca castoff at the time) - he WAS a fairly big deal coming from Barca, but he was still an outgoing Barca player!, and Silva was a nice little Spanish player on the fringes of the Spanish team, but possibly too small to make it in the PL (as was the view then). WIthout a shadow of a doubt, both have increased their profile since joining.
Tevez was also a huge signing for us and real PR coup at the time, certainly as big as Yaya, and definitely a bigger signing that Silva (I'm not talking about importance, just the marquee stature of the signing).
The pool of players that would improve City of 6 years ago was much larger than the pool of players who can improve us now. The closer you get to the top of the tree, the harder it gets to find even better players, and that is what makes it harder for us.
I also did not say it's any harder for us that other elite teams, I said it's hard for us NOW than it was then. Back then we weren't an elite team. That's why I said we are now competing with a different set of clubs for the calibre of player we would like. Once we start doing very well in Europe, things might get easier, but until then, if we want the world's best players to join us, they need some serious coaxing. We are still a 'project' on the European stage, whereas domestically, we have gone past that stage.
If, as you suggest, things are not harder, but ought to be easier for us, then we do indeed appear to be struggling to improve in a number of positions on the pitch.
This whole subject came about over criticism about how we 'seem' to be failing to land targets when in the past we managed to land them. I don't believe we are failing (we've landed 1 of our 3 obvious big targets with a second one seemingly imminent, and brought in a couple of others too), and I'm arguing are previous targets were from a wider pool of players we might have bought. The pool has severely diminished due to us being a far better team now then we were then.