SE needs to actually balance the jobs so that the difference between good classes and terrible ones is not so stark. That's the only solution. Every job should be given "main niche" (DD, Healer, Buffer, Tank) and should perform within 5% of all other jobs in the category. Pure DDs like MNK should be at 100% damage while jobs with other roles like BLU or THF could be at 95% of monk's damage.
Yes, that would mean a GOOD blue would probably outperform a monk in many situations, but a) BLU is harder to gear/play at 100% and b) it would actually BE OK if there was only 1 mnk in a party instead of 3.
Yes, it would make mixed jobs more desirable, but that's the point. And it would automatically result in more job diversity cause while it might make more sense to bring a THF than a second MNK if thief was competitive, it still wouldn't make people want the party to have multiple THFs
Currently SE MASSIVELY overestimates the value of ... well pretty much everything except straight DD power, which they constantly under estimate.