If he's already driving in the limit what is there to improve? Some people don't seem to understand that. It's like if two teammates are so good that they are very close to the limit, and then suddenly someone asks why one of them is not 0.5sec ahead. That's not possible.
But who knows how close or far away they are from the limit? It's very rare that someone get that perfect lap where they don't think they could have gone any faster, and even then there is probably still room for slight improvements and that is not even considering improvements in setup.
I think the way Stoner trashes eryeone at Philip Island every year, shows that they are not that near to the limit as you might think. It is just that Stoner somehow can get much closer to the limit at Philip Island, where as they are all pretty similar everywhere else. The other logical conclusion would be that everyone else just can't get as close to the limit at Philip island compared to every other track, which seems strange.
The natural order of things is also that eventually the current benchmark will always be surpassed by someone younger and faster from the next generation.