I think the monk complaints were mostly about perception rather than in practice.
People remember the spikes in damage. Which Mr. Mearls talks about. You remember the spikes and alpha strikes. So while the monk holds their own, they do so with consistent even damage. While the rogue does so with memorable critical sneak attacks.
Monks actually do pretty decent damage. At first level they can do 1d4+3 twice a round, for 8 damage on a good round, which is better than a fighter with a longsword (7 damagae) and close to a great weapon fighter's 2d6+3 (10 damage on average). And once they get ki and can add another couple attacks, they can double that. Which is generally even with Action Surge, if not usable more often.
Plus, since they're using multiple small attacks, when you include accuracy their average damage is higher, as they have more chances to hit and not miss.
And their damage dice goes up pretty quickly. I had a 9th level Way of Shadow in a mini campaign, and she could tear through enemies. At five 1d6+5 attacks, she could potentially do more damage than a rogue at that level, who might be doing 6d6+5. (Heck, even without flurrying, the monk's average damage is comparable to the rogue with sneak attack.)
Emphasis added.
It's actually pretty simple why are they the least popular classes: most players build their characters based on story and the character they want to play and not what is mechanically the strongest.
Paladins and bards have funky flavour (as does the monk) and the bard especially seems somewhat comedic. And the druid's role as the guardians of nature isn't always as appealing to players. Especially as they have a lot of aspects to manage, requiring knowledge of spells and animal for wild shapes. They've never been exceptionally popular.
