There is always the possibility of learning new things even for someone like Clapton, although what you learn might not 'improve' your playing. If someone like Clapton or Page decided to learn to play some Megadeth riffs with heaps of distortion they would be doing something different but no sane person would say they had improved their style simply by making a radical change like that having run out of ideas.

Thats where SRV, Hendrix etc have and advantage, they never got the chance to become boring and old and feel as though they had run out of material.