Most players IMPROV over time. He's been the same player for 20 years. Hasn't changed.
A good example of a great artists who changed himself up a bit over his career is Tony Macalpine. Went from a classical vibe to more of a fusion type deal later in his career.
Same can be said for Greg Howe going from a fusion type vibe to the more neo-classical style stuff on Ascend.
I don't think it even neciserally has to do with getting necciserally better as much as changing up your style a bit.
Then we get into the whole issue of "Oh, ____ didn't stick to their roots, they suck now."
I dunno. Yngwie is what he is, as far as I am concerned. I'll put him on for a bit when I'm in the mood for that sort of thing... If i'm not, I put on something else.