A very good point John.
When I exhibit at my local art shows I am often asked why I don't specialise in a particular subject or theme, "You would do much better if you did", I am often told by the armchair experts and weekend browsers.
I disagree. Different people like different things, so I will take pictures of anything I think may be sellable, or which I think would appeal to somebody. At a local art fair recently I sold pictures of buildings, steam trains, landscapes, places of interest, floral, dogs, travel, abstracts, and probably a few other things I've forgotten about. Of course there are many who do specialise and I dare say become experts on producing art of their chosen subject, and no doubt make their fair share of sales, and I take my hat off to them.
Personally I think I would go nuts if I could only shoot one particular subject, theme, or style!