Again, I said most fights have at least one forced GL3 drop. Some have many. There are plenty of jumps where proper timing can save it (which isn't that easy btw when dependent on the party's DPS) but thing's like Titan's Heart transition or Ifrit X's dashes just laugh at your attempts to save GL3.
Unless the MNK, the DRG or the BRD in question is considerably under-performing here, DRG + BRD still matches or exceeds MNK. That's a problem. When the party has 2 or more BRDs, there would be no issue. However, currently it only takes 1 BRD in the party to put DRG's effective DPS at or above MNK's.
Not many fight's at endgame where the tank is taking loads of magic damage... at least not yet. Even that aside, the INT debuff isn't particularly strong. Similar to PLD's STR debuff, it significantly pales in comparison to any % damage based mitigation option. If by some chance a fight emerges when that INT debuff is enough to keep people alive, perhaps. That's a longshot.That being said, monk is a clear cut winner in the progression department. When doing progression, squeezing 5%-10% more damage doesn't often make or break a fight. Things that make progression fights easier are tank / healer cooldowns and gear. I think often people forget that dps requirements are not very high in ffxiv. Monk has a 2 minute, 20% healing increase to the entire party. This can be used solely in the tank's cooldown rotation, or even after the party takes a lot of damage. The int debuff is also useful if the boss does any sort of magic damage. These 2 skills alone make the monk shine over dragoon, where dragoon's only benefit to the party is giving 10% more damage to the class that purposefully nerfs its own damage to make support songs.
DRG can also get mantra. So can BRD. Sure they're not as potent, but typically enough. Mantra is a crutch option. It's not something to rely upon anyway.
For DPS roles, DPS is of paramount importance. Your job is to maintain as high DPS as you can whilst avoiding mechanics and staying alive. Talking about progression in a game riddled with DPS checks of various degrees, you mean to suggest that 5-10% more DPS is insignificant. Really? Personally, I think it speaks volumes that you would even say that.
I can see where you're coming from though with the progression argument if I overlook that bit where you undervalue DPS in progression. MNK can indeed be more effective in progression than it is in farm-mode (assuming you've got an amazing player who will adapt to new mechanics quickly and not drop GL3 every opportunity). DRG on the other hand isn't any less effective in progression. It's more consistent. While we're on the topic of progression, BRDs are actually quite effective. It isn't a stretch for teams to run 2 BRDs for song flexibility in progression, among other reasons. At this point, DRG still takes the edge in progression.