It's not as easy as you may think... the only reason they get the high ranks is because some of these players have been playing the game (with each consecutive build) for months, literally months, and have a good idea of how everything works in order to get your style up.
This reminds me once when DMC4 wasn't out yet, and some dude from Capcom did a demo walkthrough of the game, the guy was so good that he got S-SSS ranks in all of the fights throughout the demo. Granted that in DMC3 and 4 it is harder to keep SS and SSS ranks because they actually go away if you spend too much time without filling your meter, while in DmC you have three different bars for S, SS, and SSS that will not bring you down once the bar is depleted... unless you get hit.
But for the most part... whomever said it was hard to get an SSS in past DMCs? It wasn't...