Yo, I'm sorry but from that screenshot you posted, it seems this has NOTHING to do with parsing and everything to do with the tank complaining that the other tank wasn't doing mechanics properly.
And even if parsing didn't exist in any shape or form (unofficial or not), I bet this guy would still be as "toxic" as you think he's being. So please... knock it off with the "OMG PARSERS ARE BAD CUZ JERKS EXIST" attitude, and just deal with it like a mature person. If you see someone is being a dick about something, either kick them out or go find a new party with the people who agree with you.
No. That's what a tank with the best gear and who uses pots ends up doing. Higher if they're optimizing party buffs. And even higher if they end up with a lucky run where they landed lots of critical/direct hits. But expecting tanks to do over 3.5k is borderline elitism. DPS should easily be staying above 4k though, but factoring deaths it is not unusual to see them drop to the 3.5k-4k range.
Not gonna lie... I laughed way harder than I probably should have at this. It sounds so mean but at the same time it would get people to open their eyes. Thing is, I believe if someone isn't doing that great, there's a high chance they're still learning the fight. So it's not fair to immediately point fingers and kick people off just based on their parse damage. I remember when I was learning A10S, I was in a lot of parties who kept yelling at me for staying in Defiance too much. When the fact is I was still unsure of how the fight went, and I had died lots of times before due to not having enough HP. So at least give people the benefit of the doubt if they're not doing as best as they can. Sometimes it doesn't mean they're bad, but just that they haven't done the fight as much as you have.