In classical chess his last fifteen round robins have all been very strong super tournaments, and he has won ten of them and finished second in the remaining five. During this period he has also played two title matches and won both, scoring the highest percentage (65) in a title match in more than a hundred years in 2013.
These achievements will not be easy to match for future top players. Just winning both the rapid and blitz World Championships in as strong (and rather short) events as they were the last time is very difficult even for the best players. Some players of the past may well have scored similar results if all these events and rating lists existed then, but doing it in the future will certainly not be easy.
No, breaking his record will not be easy but it will certainly happen.
No records stand for long in sports.
I remember when I was a kid, Bob Beamon broke the existing long jump record by something like two feet (more than half a meter).
World record progression for the Long Jump (men).
People argued whether or not the altitude had something to do with it. They said it would never be broken. Few ever even came close. At any rate, the new record stood for more than 23 years but was finally broken.
No record stands forever.
Given that chess rapid and blitz ratings have just started to be recorded, there is no reason to believe that future world slow chess champions won't also be blitz and rapid champions.
Come back in 100 years and let us know.
Magnus Carlsen is not only reigning World Champion in classical, rapid and blitz, he is also the highest rated player in all three formats. In classical he has a lead with more than 60 points, in blitz with more than 90, while it's just over 5 points in rapid.
In classical chess his last fifteen round robins have all been very strong super tournaments, and he has won ten of them and finished second in the remaining five. During this period he has also played two title matches and won both, scoring the highest percentage (65) in a title match in more than a hundred years in 2013.
These achievements will not be easy to match for future top players. Just winning both the rapid and blitz World Championships in as strong (and rather short) events as they were the last time is very difficult even for the best players. Some players of the past may well have scored similar results if all these events and rating lists existed then, but doing it in the future will certainly not be easy.