Metajedi is 100% on point. Players need to pay attention to counterclasses a bit better.
A lot of people are suffering greatly from the dunning-kruger effect, where they think they are super amazing players when they aren't. People under the dunning kruger effect tend to believe that if they fail at something, it has to do with external conditions rather than their abilities. So if you're a sorc and you get rolled by a slinger, it's not that you played badly or didn't have cooldowns or whatever, it's "OMG SORC UNDERPOWERED SLINGER OP BIOWARE PLZ NERF QQ I UNSUB".
That said, the tanksins passive damage mitigation was comparable to juggs, but their active defenses were superior; all the while their damage was also superior.
So the rebalance is on the PTS. We can fight it out on there, and find out if the adjustments were heavy handed or not. Bioware will take a look at the numbers too.
All I know is that if anyone actually threatens to unsub or whatever because of this, they are petulant baby children.