All of this is from the perspective of arena pvp, as I don't have enough experience in zone to have a valid opinion.
IMO masterminds bring too much passive disruption to a game to justify being a legitimate offensive threat. Purely by the mastermind existing, metaphorically being afk in the middle of outbreak, the other teams offence has to slow down significantly, this is currently balanced by the MM not being able to contribute a lot to a spike. This sounds like a fair trade off to me. And please don't say "just tilt your camera up", just because some people seem to have their space bar taped down doesn't mean this is a legitimate way to "counter" the disruption that MMs bring.
Essentially when there is an MM in the game, the entire strategy of the game revolves around either dealing with, or playing around the MM. You target people who play away from the pack, you try (and usually fail) to catch the MM off guard to get a kill and hopefully 30 seconds of peace before they respawn, you position yourselves such that people have to leave the safety of the MM ball of doom, you try to find ways to pressure the limited offence (this is a big one that I really don't want to lose).
I don't want to name drop because I don't know if they want to be forced into this discussion, but when I hear from someone that I think most everyone would agree is a top player, on a team filled with people generally known to instantly lock:
I can't help but feel like MMs are doing their job just fine, hell I actually think its a good thing that they can slow the game down. 8v8s at the moment are a bit of a mess in that for standard jump teams, you can't really pressure an offence. The fact that things like MMs exist that can slow this down and make a game more strategic is a good thing imo (I also think that 6v6s should be the standard format for exactly this reason, but thats a topic for another day)
Buffing MM damage would be like saying a grav/ta controller needs to do more damage. Their role is de/buffing and disruption, if you want to do offence and de/buffing, play a corr/def. Imagine we buffed MM capabilities to match that of a defender, why would anyone play defenders? You get the same (reduced) buffs/debuffs, the same damage, but you also make your entire team harder to kill simply through your existence. I hear you saying "well it would be less than a defender obviously", but how much less is that in reality? Defenders are already on the cusp of being too low damage IMO.
I keep seeing this rhetoric of "MM's are only viable with a kin and this is a bad thing" and I just can't agree with it. Firstly IMO MMs are perfectly viable without a kin, they are just far less fun as you can't move around as much and yes, teams can abuse this lack of mobility, however I can't help but remember certain turtle strategies that seemed to work perfectly fine (but again weren't fun)... I also don't think this is a bad thing, I feel like bringing a buffer to cover for another characters weaknesses is perfectly fine.
Don't get me wrong, I don't think that playing the afk "I exist" playstyle is fun, but fun and balanced aren't the same thing. I would like to see any changes to MM offence be counteracted with nerfs to their disruption/defence, but that would also have us lose out on what makes MMs unique at the moment.
I feel like I should also point out that there is more to arena than just 8v8s, and I seem to remember a rather handsome MM players team winning a 2v2 tournament by being basically unkillable while afk with healing aura on auto, does that sort of playstyle also need to be a damage dealer? I don't think it does.
I guess what I'm trying to say is, if you want to play a high mobility offensive threat, maybe don't play the AT that is slow and low damage
sorry for the rant