It depends. On the short term, anyone could have matched Ponting's runs in this series. Peter Siddle is just 4 runs short.
And in the last two years, a lot of actual batsmen could probably have picked up Ponting's runs at an average of 38. We ditched Marcus North for not being good enough, but in his tenure, he made more hundreds than Ponting.
Sure, there's no replacement for Ponting circa 2002-2006, but if he himself is merely half that player, he's not much of a stand-in either.
But of course, it is possible to carry a player until he rediscovers form. What matters is the team. Players should never be bigger than the team.
Ideally a team has the structure to carry a batsman or even a couple, but with recent selections, this one has lost that. The selectors overlooked in-form replacements for Katich and North. With Ponting and Clarke at the moment, that means more of them are out of form than in form.
It is interesting how supporter pressure has fallen hardest on the senior players, while the neutrals seem to be their strongest advocates. I think it shows that while it is hardest to see the big picture up close, nobody else has to deal with seeing the gory details.
