True all that. The successful operations that will hopefully be here 50 years from now all own their own track and typically (though some exceptions) run smaller power (no Mountains, Northerns, Berkshires, Articulateds). A few representative operations which by no means is a comprehensive list:
Valley Railroad
New Hope & Ivyland
Strasburg Railroad
Great Smoky Mountains Railroad
Everett Railroad
Tennessee Valley Railroad Museum
Mid-Continent Railroad Museum
Lake Superior Railroad Museum
Boone & Scenic Valley Railroad
Abilene & Smoky Valley
Durango & Silverton
Cunbres & Toltec
Niles Canyon
Even the Grand Canyon Railway runs a light Mikado as its largest unit, and then only occasionally.
Granted a few places will be running larger steam on a more frequent basis. Reading & Northern will run their Northern 2102 a bit more frequently than most large power can get out, because they own the track, as will the Western Maryland Scenic with 1309 (although it will be interesting to see how fuel and maintenance costs compare to a 2-8-0). And the Black Hills Central run the 44-inch drivers off their logging Mallets out of necessity due to the terrain.
But even Jerry Joe Jacobsen ran his light Pacifics the most frequently when he owned Ohio Central, and places like the Cuyahoga Valley Scenic Railroad and Steamtown might run steam more often if they had smaller power to run. (Though the 765 visits are fantastic!)
The big units we all love - 614, 765, 261, 844, 4014, 3751, 4449 all get out sporadically, and for that we are most thankful.
Reliable lighter power for the operating museums will continue to be crucial to keep steam operating in to the latter half of the 21st Century.