You can take the discussion broader. See the attached table.
The peak years for slam winners are from ages 21-27 (data through Wimbledon 2013). Nadal has followed the script, winning early and having his best year at 24, which is incidentally is the most common age for winning a slam. There is a dramatic drop off in the number of winners who are older than 27. Going by this, you’d say that Nadal’s chances of winning 5 more slams are not very good.
But, and it’s a huge but, there are two factors that could alter this. One, the cohort group currently dominant is 26-28 years old, which is about 2 years older than usual, so 29 could be the new 26. A year ago, you could say this won’t apply to Nadal because of his mileage but this is no longer true. He looks in the best shape of his life.
The second, and even more important, factor is that the cupboard is bare. History says that the maximum winners of slams come from ages 21-26, so we should have some players in middle of that age group who are winning or, at the least, going deep in the slams. Unfortunately, there are almost none. In the top-16, there are only 2 players – DelPo and Raonic – in that age group and neither is a serious threat to win – Raonic has never been and DelPo not for 4 years. The others further back are Janowicz, Dolgopolov etc – all works in progress.
These are unusual times. Consider a thought experiment. If entry to slams was capped at age 25, could anyone with any confidence predict the winner or even the final four of a single slam? Today you can get 3 out or 4 correct most of the time.
Or, without checking can anyone tell the best 4 clay courters below the age of 24? I can’t. Would any of them have a hope in hell against, forget Nadal, but a 31-year-old Ferrer? These are very unusual times, in terms of the sheer lack of quality in the age group 21-24, which are traditionally the most productive years.
So, in a way this is the golden age with Roger, Rafa, Novak etc. but we are heading for a really weak era after that. There are no freash challengers out there. So, yes, 5 more slams are possible.