There are landscape photographers, people photographers, those who shoot sports or glamour, street photography or stil life, nature and so on. There are photographers who shoot whatever happens to be available because it's the process of photographing they like, the challenge of working out a good picture whatever the subject. This would seem to fly in the face of advice to photograph what interests you.
I would say though that the majority of great photographers have been fairly versatile, photographing people and places, things and even critters. Ansel certainly did a fair amount of portrait work and in the early days made his income from his industrial work. His books include back yards, water towers, porches and people. He may be known to the general public for his mountains, but that certainly doesn't define him or his work.
Edward Weston was very versatile, photographing nudes and portraits, landscapes and industries, still lifes and vegetables, toilets and taps. Michael Kenna is known for his moody misty square simple landscapes, but one of his most striking series was on the cooling towers of a British power plant. John Sexton may be about as pure landscape as you can find, yet one of his three books is industrial. David Plowden is known for his recording of small town America, yet has done landscapes, industries, railways and ships. His railway pictures usually include workers while his town pictures generally don't.
Certainly my own experience is that light is light and composition is composition, whatever the subject matter, and that while there may well be some technical issues to switching subjects and even a learning curve, it makes sense that good photographers would apply their talents to more than one subject.
I've written in the past about switching subjects to get out of a rut but maybe you should be doing so anyway. I can't prove that it's good for you, but it makes sense not go go against what better known photographers than you or me have done.
I see no difficulty with photographing two or more types of subject simultaneously - well, in the same week at least. Some projects can take many years and a photographer could very easily have ongoing projects in landscape (my badlands series) and industrial (my independent machinery series).
I would predict that had someone like Arnold Newman or Robert Mapplethorpe applied themselves to landscapes, it wouldn't take them long to become very good at it. They had the eye, knew what surfaces photograph well and knew how to compose. Robert would want to know how to adjust the light and Arnold would ask for the subject to move over a bit, but I don't think they'd have that much trouble catching on.
What about you?