I think his teacher meant that in 20 years there will be more guitar players, which will be seen more (technically) proficient, popular, better tonally etc. This disregards his enormous contribution to the classical guitar as a whole, but i see the point.
When something becomes popular, the pioneers will sooner or later be "forgotten" as such and compared to players way after their own time without any context to time, overall situation of the instrument etc. People's memories are short and there already are few generations after Segovia who never witnessed him at the height of his fame and can evaluate his playing and tone from old recordings and youtube videos.
An apt comparison would be Jimi Hendrix - someone could argue that any modern shredder can be technically more proficient and faster than him, but before him there were no (electric) guitar heroes. And the things he did with feedback are copied to this day, similar to the techniques Segovia used and made popular.
To me when someone talks about Classical Guitar, the first player that comes to mind is Segovia. Immediately after that there's Williams, Bream etc. but more or less Segovia still is "Mr. Classical Guitar" (like Chet Atkins is Mr. Guitar