IMO "traditional" landscape has been beaten to death. Waterfalls are nice, but I have yet to see a better shot than Caponigro's Glencar falls. Who can make a better shot than Adam's "Clearing winter storm" or any of the Barbaum landscapes? Few of us have the time, money and luck to be at the right place at the right time.
OTOH the current successful landscapes are not the dramatic, Wagnerian ones, but the minimalistic quiet ones. Take a look at the pictures from David Fokos, Bill Schwab, David J Osborne, and you will see what I mean.
As a magazine editor get used to seeing the same thing over and over and over. Unfortunately there are many photographers out there who are technically superb but have not found their place in photography or have fallen on a rut. It is easier to keep on doing the same stuff you have been successfully doing in the past than moving out of your comfort zone. As such I am sure you will be getting a lot of waterfalls, water running over rocks, etc, etc.
I think your friend photographer misspoke, collectors no longer buy the "traditional" landscape unless the photographer is one who is well known. But I think they buy the new breed of landscape like those of whom I mentioned.