It takes the Hendrix and Parker to tear down the old ways and start something new and then it takes a generation of their followers to stay with their newly created tried and true and perfect it until somebody else gets bored with THAT and tries something radically newer yet.
Ray, thats not quite what I'm saying and I don't think its what Martin Parr is saying either. Some of the examples that he gave were of people who are radically different from the mainstream already, but his argument is that in itself then becomes a cliche. Certainly with the examples you give, once Charlie Parker developed bebop he didn't move away from that. He didn't start playing free jazz for example. Hendrix, once he'd got his sound and style sorted, again didn't suddenly start doing something else.
Parrs argument has nothing to do with moving away from whats gone before, its saying that once someone works in a certain way, no matter how mainstream or avant garde it may be, that then becomes static. And if something is avant garde when it starts, then by continued repetition and imitation, as he sees it, it then becomes a cliche. For instance if you are a disciple of the Dusseldorff school then portraits consist of blank, featureless, unemotional expressions. A quite radical departure from how portraiture was mostly done before. His argument is that once everybody starts doing that, it then doesn't become radical anymore, but the norm. A cliche.
His blog piece is about how he feels about that. He says:- "After 30/40 years of viewing our work I have come to the conclusion that we (fine art and documentary photographers) too are fairly predictable in our work" Radical, maybe. Avant-garde, maybe. Mainstream - probably not, but still predictable.
Because of the highly stylised and often proscriptive nature of many of the examples he cites, I can understand that. If all you ever photograph is blank, expressionless, non-emotional portraits of people then I can see how that gets pretty difficult to take after a while, and if you are a college tutor then looking at portfolios which consist of nothing but that must get pretty tiresome. But that depends on why we take the pictures that we do. Do people take pictures in a certain way because they want to, or because they feel they are obliged to because that is the current fashion in fine art photography and they want to be thought of as "fine artists".
I don't just have objections to Martin Parr, I dislike much of modern fine art photography, precisely because it is unemotional, lacking in passion, and often cruel, shocking and deliberately "unaesthetic". The notion that an "artist" has to constantly change to avoid "predictability" is a value judgement. One persons "predictability" is anothers "consummate practioner of their craft". If you go back to Charlie Parker, bebop was what he was striving to achieve, and was the pinnacle of all his years of playing. The notion that he would suddenly decide, "I'm getting predictable, time for a change" would be valid if he felt he was getting nothing from his music anymore. But if he decided that he still had much to explore, then there's nothing wrong with that.