FWIW fighters are the most popular (another suboptimal class)
Given that DDB data also shows most characters never make it past 5, which is where Fighters shine, this is perhaps unsurprising.
Re: Monks etc. I think part of the issue is that how much a class is played has relatively little bearing on whether class is particularly well-designed.
This is easily demonstrated with MMORPGs. Very often classes that are poor performers on metrics and have serious design flaws are actually extremely popular with people playing more casually (which is typically "the majority". That doesn't mean that the design flaws don't exist or don't matter though - just that they don't seriously and immediately impact play. What is notable is that if they do get improved, their numbers do go up - but only a little bit.
You seem to be trying to say "If people don't think they're flawed they're not flawed". That's obviously irrational and non-factual. Especially as you're going with "people play them" as proof they're not flawed. Again, easily demonstrated by MMORPGs and the like. Hell, I've seen MMORPGs where the most well-designed classes barely got played compared to some of the worse ones!
Hell, 2E is a great example. Thieves were a total
disaster of a class in 2E, a real mother-may-I nightmare. But loads of people played them!
A better argument would be "It doesn't matter if they're inferior, people play them anyway!". I mean let's all be real, in 5E, below level 10, the difference between the most effective classes and least is like 40%, at most (barring serious optimization). That's amazing. Only 4E did better. Again this doesn't mean the flaws aren't real, shouldn't be addressed, just that their impact is limited.
(And with Monks btw I'd love to see how popular they became if they stopped them being Shaolin Monks and basically made the Mystical Martial Artists - they'd probably become the most popular class!)