Regarding modern landscape photography for sale, I cannot imagine making a (decent) living off non-commercial/magazine "art" landscape photography alone, not today. Short of working for Nat Geo, it must be almost impossible, as an independent photographer. The market is hypersaturated and the buying population is bombarded with images.
Plus, as an analogue person selling individual analogue prints, there is a lot of ongoing overhead expense and you're up against people who use digital and have much less overhead in the long run. Some will say that the upfront premium for high end digital gear equalizes that, but I suspect that $50k is a drop in the bucket if you consider the cost of arranging access and travel working in the field at the kinds of unique sites that will sell magazine images. All of us landscapey analoguers have run the numbers and concluded that $50k buys a vault of medium format velvia, but... velvia in your fridge is not worth as much as velvia at a base camp in the Karakoram with a team of guides and sherpas We're probably talking $50k just for one 1-week trek.
Now, on the other hand, I can easily imagine making a living off of people who are trying to make a living off of their photography. There are agencies and online picture-pushing services like flickr etc. that profit handsomely from people's dream to be the next AA or Rowell or whatever. That's where the money is- servicing other people's passions!
Fortunately for me, these financial concerns don't enter the considerations, I simply do what I like and work at the rate that my (modest) fun-budget permits. So basically I am one very lucky bloke. I have been super-blessed by jobs that allow me to go amazing places and slip in a little photography on the side. One of these days a meteorite is going to slam into me and my gear and average things out.