It’s either all supports are ‘aggressive supports’ or none are. If you mean aggressive support as a support who’s supposed to flank the enemy and get gold damage, I don’t think any are designed for that. Mercy is in fact fairly aggressive with tempo rezzes (Rezzing some one instantly to keep a snowball going without losing it’s momentum), damage boosting and pocketing a DPS (because she’s focused on damage while doing that), and even Valkyrie is one of the best support ultimates for being aggressive with mass damage boost, or even her infinite ammo blaster if you really wanted to.
Mercy doesn’t need peeling because she’s able to consistently rely on her passive healing to heal her. She’s not like Ana who needs to rely on her nade for self healing, Brigitte and Moira who need to rely on damage or healing orb for Moira, Baptiste who needs to rely on his regenerative burst, or Zenyatta who needs to rely on his shields to recharge. The only support who’s comparable to Mercy on the level of self sustain is Lucio. Mercy doesn’t need to use a cooldown or ability to heal herself, as after 1 second of not taking damage she’s able to begin healing herself. For that reason, Mercy is able to in fact make aggressive plays without having to worry about using a cooldown knowing she just has to hide for a few seconds to be back up to full HP or near it. At the end of the day though, Mercy is still a support. She needs to be healed and shouldn’t rely too much on her self healing. Don’t think just because you’re playing Mercy you don’t need to be healed. I think Supports are the most important member on a team to take a health pack (other than Tracer and Wreckingball, sometimes other dive heroes), even if it’s Lucio or Mercy. Supports can heal their injured teammates, but those injured DPS and Tank can’t really heal their supports unless they’re playing Soldier or in some cases Sombra (or Echo if she duplicates a healer). I understand your comment said she is not meant to need much peeling, which on paper is true, but this bit of my comment was simply meant to show that what goes on paper doesn’t always transfer to an in-game environment.
Mercy is in the top 3 for best peel supports. Is Brig really better for peeling? It depends. Let’s say you’re defending first point of Volskaya on Brigitte. Your second support, Ana, is playing somewhere on the high ground above Mega. Let’s say enemy Sombra sneaks past your team and hacks Ana and begins shooting her. You, on Brigitte, are playing in between point by the crates and near your tanks which are playing from the main choke. Let’s say there are no comms meaning your Ana does not ask for help over voice chat (since every team is different and you can get Ana players who don’t ask for help over voice on Mercy but do on Brig allowing for a better peel, and vise versa). As Brigitte, you are probably distracted healing your frontline or triggering inspire, but you hear Ana’s voice line when she’s being attacked, or you hear Sombra decloak or hear her shots. You turn around to help your Ana, but from your position, it’s VERY hard to throw her a repair pack without her jumping down. If Ana jumps
down, you heal her and more than likely Sombra translocates away. For both scenarios, I will say Ana doesn’t jump down. You probably manage to throw her a repair pack before she breaks LoS and you go upstairs. You heal Ana and do enough damage to Sombra to force her to translocate away. Ana survived, but it was a long time where your team was receiving no healing and it’s possible another teammate died in between.
Now, same exact position with Mercy. As soon as you hear the Sombra voice lines or Ana’s voice line for when she’s taking damage, If you’re playing in the same spot as Brigitte, it’s very possible you won’t find an angle to GA or heal Ana and will need to walk up the stairs aswell. In that case, Ana would probably be dead since she hadn’t gotten the extra bit of healing, but if she’s not, Mercy can out heal Sombra’s damage and force her to translocate away. But, here’s the thing. Mercy is flexible enough to not need to play pushed with her team. If she’s across the point on the ledge by the mini pocketing a DPS, she’d be able to fly straight to Ana and get there much quicker. Even if she was playing by her team like Brig in this scenario, there’s still a way to super jump off of one of her teammates and get to Ana much quicker as well. Now, I’m not going to give another scenario where Brig may be more ideal, but this was just to show it’s never set in stone which support is better at what; especially when it comes to peeling. If Ana was closer to her team or playing low ground, Brigitte would be able to keep inspire on her team while helping Ana, so probably would be better than Mercy and have neither healers focused on healing. Brigitte doesn’t need to be healing to actually heal which would be important if Ana was playing closer to the team or if Brigitte was playing with Ana, but Mercy is able to get into tons of different locations where Brig can’t without wasting time getting there.
Comparing current Brig with current Mercy is like comparing dogs and cats. Both do different things, and both are better or worse depending on team composition and overall team positioning. Neither Brig or Mercy is significantly better than the other. They both have weaknesses (Mercy doesn’t do well in comps without ideal damage boost targets like Pharah, Ashe, Hanzo, Soldier, etc. aswell as comps that are grouped together like brawl and bunker unless played with a Bastion, whereas Brigitte doesn’t play very well when every one is spread out, and doesn’t do well when the enemy play from a distance), and they both have many strengths as well.