You're right about monitors, but the other thing is aspect ratio: Old monitors were square, playing old games on modern monitors stretches it out comically without somehow forcing it to stay square--and thus waste 1/3 the screen. Are LCD THAT much cheaper to produce than CRT?
I agree about profit margins, not sure about attention spans, but probably true. PC players would have a MASSIVE advantage on any control-scheme that allowed it. 115-button controller vs. 12? The last FPS that allowed cross-platform play to my memory was Shadowrun--and PC players THOROUGHLY stomped PS players! Therefore they had to keep us separated. Otherwise people would stop buying consoles and realize the superiority of PC at least in controllers, if not in the rest of the mechanics. Another advantage (For the company) of dumbing down a game enough that a console could play it, it makes it simpler and faster to code on all platforms. How fast are they churning out sequels now?