Remember that one of the all-time greats, Andre Agassi, used 20 x 22 string patterns.
Also, Federer, even though he uses 16 x 19, uses a 90 si frame, which makes a 16 x 19 rather dense, actually. Compare a 100 si to 90 si frame, 16 x 19 string pattern will much more dense in the smaller frame.
Folks who use the venerable pt57a in a 16 x 19 drill pattern also have a rather dense string pattern as well, as the si is not quite 98, but closer to 95 or 96.
Back in the day, it was pretty much impossible to have an "open" string pattern with a frame size of less than 90 si. So all those 88, 85 and 80 si frames, regardless of their string patterns, because they were so small, the string patterns were really considered "dense".
IMO, it wasn't until the advent of really huge frames (100 si or bigger) where "open" string patterns really came into play.