Maturity of a technological system to me implies a plateau of design, but that's not the case in missile guidance. Current generation systems are significantly more advanced and more capable than previous generation systems.
Yes, sorry for nitpicking but I just think that the word choice was perhaps not good. I'd think modern missiles means something much more recent easily, when this is talking about something practically existent for the whole development time of missiles. Sidewinder development was started in 1946.