In sport, standards generally rise over time. Whether it's the physical or mental conditioning of the athletes involved, the strategies deployed, or the standards of equipment and technology, the push for higher, faster, stronger courses through the veins of competitors.
It's reasonable to say that the current crop of grand prix drivers are, in absolute terms, the best in history (which is why, when attempting to compare eras, all drivers have to be judged according to the standards of the times). But contrary to the trend, you can argue that today's drivers might never be bettered.
The reason lies in the peculiarities of contemporary grand prix racing, specifically the constant battle to keep costs under control. This has led to, since 2003, a gradual increase in restrictions on testing to the point where, in '09, in-season testing was banned outright.