I'd say they are no longer a great martial artists, at least if fighting was the goal of there art. They might be a great teacher, but no longer a great martial artist. In the same way Joe Fraizer is no longer a great boxer, lots of knowledge yes, still a great boxer - no. Same with any other sport or physical activity. Is Bobby Hull still a great hockey player? No, but he was 30 years ago.
On the other hand I'd say someone like Jackie Chan or Jet Li are great martial artists, even though they never fought, and never claimed to be fighters. What they did was still martial arts, just performance baised, and they where great at it.
There are different aspects to martial arts, fighting is only one path. Performance is another, preserving history another, some are even in it for spiritual reasons. So to each his own.
On the other hand I'd say someone like Jackie Chan or Jet Li are great martial artists, even though they never fought, and never claimed to be fighters. What they did was still martial arts, just performance baised, and they where great at it.
There are different aspects to martial arts, fighting is only one path. Performance is another, preserving history another, some are even in it for spiritual reasons. So to each his own.