You aren't disagreeing with me about this; you're disagreeing with the researcher about his own research.
Whatever your opinion about "four or five years of full time professional experience" that is simply not what the 10,000 hours factoid was about.
Anders Ericsson tells us "the number of years of work and leisure experience in a domain is a poor predictor of attained performance" and what he's actually talking about is deliberate practice -- "the accumulated amount of deliberate practice is closely related to the attained level of performance of many types of experts".
I know what the Ericsson work is about; I've read his work, and a couple more books based on his work. What I am arguing is, in many professions four or five years of work (under certain conditions) may constitute "deliberate practice," if you bother to understand what "deliberate practice" means. A reporter, for example, who is regularly pushed by a city desk into newer, harder reporting fields and writing assignments, and who regularly has those assignments critiqued,is experiencing what amounts to deliberate practice, and at one time, this was a standard way of giving a talented reporter advanced training. On the other hand, some (perhaps most) don't get this kind of attention, and they don't improve.
You said that "the 10,000 hours applies to 'domains where performance consistently increases' and that isn't obviously true of photography," and that was the statement I was disagreeing with -- the "not obviously true" part. I think it may very well apply to photography, so I was disagreeing with you, not with Ericsson.
When Ericsson said that the number of years of work was a poor predictor, he's talking about large averages. If you look at 10,000 musicians, the number of years they've been playing is not a good predictor of performance because the harsh fact of the matter is, most musicians are not that good. But the 10,000 hours of "deliberate practice" is a good predictor of peak performers. I'm saying that the same is true of talented lawyers, reporters, TV commentators, artists, preachers, politicians, etc. They seek out the equivalent of "deliberate practice" and that's what distinguishes them from others in their fields. The other thing to understand about Ericsson's theory is that it's the result of observation, rather than deliberate experiment. So they did not have a bunch of talented violinists divided into two equal groups, one of which was allowed to practice 23 hours a week, and the other of which was only allowed to practice ten. These were self-selected groups, and so the outcomes were derived by exactly what I'm talking about -- observation of the fact that people become peak performers through "deliberate practice," but that's mostly of their own devising, and often, from work.
Also, the 10,000 hours is not a "factoid." A factoid is an assumption or idea of story or myth that is repeated so often that it's assumed to be true, where there is no evidence that it really is. The 10,000 hours idea is the result of a serious study. (The word "factoid" was invented by Normal Mailer in his book about Marilyn Monroe, in which he discussed stories about Marilyn that everybody assumed to be true, but had no basis in fact -- so these things were fact-like, and people treated them as if they were facts, but weren't facts.)