Just because this has been true in the past, doesn't mean you can extrapolate that indefinitely. Eventually there has to be some threshold where jobs do get massively affected. The car didn't make horses go extinct, but it certainly reduced it's peak population by more than 90%. What would you have to see that would make you worry about the ability for most artists to make money?
This is a good question as most of the value added by automation only goes to an ever shrinking part of the population.
I don't think it is sustainable at all, like you said. Like anything that becomes mostly software based , you don't need a big amount of people to serve a big market.
This problem is not limited to art though, we are heading for big political change in the future if things stay as they are and the benefit of automation aren't distributed.