So I was thinking about this today, and for me at least, it seems the buff was a super good idea, here's why:
Sej was a tank, she could CC people but she couldn't actually kill them if not fed, but they could kill her. Now, she lost some CC and got damage, so now she can kill squishy junglers like Kha'Zix, Kayn (and Blue Kayn), Feedster Yi and others that are squishy but deal a lot of damage, and they can't kill her, so to prevent yourself from dying to her invades, more tank junglers or at least tanky toplaners should come out in the near future in the jungle to prevent her from snowballing.
Although tanks do have damage, some more than they will ever need, it's much lower than what the squishy stuff has, especially early game, when it matters a lot to get the snowball going, but by having more tanky stuff around the jungle and maybe even top lane, the general damage will go down a notch. How much, it's unknown, but it's still less than now.
So basically, looking at the bigger picture, Sejuani's buffs should (at least on paper) lower the overall damage of the game by shifting the jungle meta to a more tanky one.