I'd say it's a bit of both, to be honest. Between )7-09 we had similar corner takers but scored a fair few from set pieces, but we had two of the best aerial threats in the league in Vidic and Ronaldo, while Brown was a regular and he was pretty handy too. Over the last 2 seasons it's been clear that our delivery, from corners in particular, is poor. But the demise of Brown and the loss of Ronaldo probably plays a part in that. Other than Vidic, we lack any real offensive aerial threat.
I realise that the World Club Cup isn't exactly the defensive benchmark but one moment sticks in my mind when we played Japanese team. We had a corner, they doubled up on Ronaldo with their two best set piece defenders and Vidic beat his slightly weaker marker to score. That's obviously an extreme example but it bears contrasts with what we see now, where teams can minimise Vidic's threat without having detrimental effects elsewhere.