Interesting video exploring why WHM might be the most popular healer despite its shortcomings. https://www.youtube.com/watch?v=_zMG-o_w0aM

tl;dw: This person believes WHM's popularity isn't because its straight forward or easy or powerful but because it's simply the only option during character creation and by the time you get more options you're supposed to either a) play a DPS for 30 levels (which the character creation doesn't really explain), b) downlevel 20 levels while the story is getting good c) you're already familiar with WHM for 70 levels by the time Sage rolls around, d) WHM doesn't teach about oGCD healing which other healers rely heavily on, making the transition in playstyles harder.

Regardless if they are correct or not i do think that giving only 1 healer option during character creation is bad game design. Scholar should be available at level 1 too and frankly SCH and SMN have so little in common with each other these days i wouldn't mind separating these into individual classes.