On the contrary...tone is one of the most important things for a classical guitarist (actually...any musician) to develop and maintain. With classical guitar moreseo than a lot of other instruments, technique really dictates tone and Segovia's technique dictated sloppy fingerings, a tendency to produce buzzy bass notes, and a tone that even Segovia scholars aren't all too fond of.
The psychological aspect of it is debatable. Having known countless musicians and being a full-time musician myself, I've never seen someone who develops that kind of ego go anywhere but downhill. Once somebody gets it in their head that they are the best, the healthy competitiveness (sometimes competing with others...sometimes competing with themselves) that drives us to constantly improve and develop as artists is no longer present and there's no longer any need or desire to be anything more than stagnant. In that sense, the complete opposite of Segovia would be Allan Holdsworth. The guy is one of the must amazing musicians to ever pick up the instrument and the guy has a borderline-unhealthy disdain with his own playing....I've seen bootlegs where he'll go up to the mic after the first song while everyone is still picking their jaws up off of the floor and apologize for sounding so terrible....despite the fact that the people watching him are convinced that they have seen a true vision of god. Coincidentally (actually...not coincidentally), the guy has been getting better and better for over 30 years and counting now.