Riot board-post: "We want there to be ONE right mastery combination for each champion."
Fans: "They add so much variety. Look how I can have choice A) or choice B)."
With the old masteries, which had a LOT more alterations to utility and functionality, there were WAY more choices that could be made. Hell, the fact alone that not putting all 5 points in a 5-point mastery or that you didn't even necessarily needed to skill a tree down to the very end dwarves the number of possible choices with the keystone-trees.
Look, might very well be that Riot HANDLES the smaller, simpler keystone-system better. And yes, they MIGHT even be outright better-working than the old trees ever did (ignoring for a moment the reasons behind that).
But I cannot imagine the amount of Koolaid you'd have to drink to look at a system that gives you roughly TWO choices (let alone viable ones) for any champion and call that system "divers". That's like a restaurant replacing a buffet with a two-entry menu and getting praised for "Adding so much stuff".
If you're doubtful about that, ask yourself why, to describe the skillpoint distribution, all you need to say is the champion and the keystone. Lucian - Fervor. Jax - Thunderlords. Brand - Deathfire. If those trees allow for SO MUCH diversity, why can I describe every distribution model with two words? Hell, the perk system in WoW requires more description.
So yeah, diversity is certainly not a good starting block to highlight the possible benefits of keystones.