I agree tanks shouldn't do as much damage as dpsers should. There are many variables for this though, and we have to decide where we're balancing it.
1) Gear. In randoms, tanks and dps can have wildly different gear and specs. An 840 tank with perfect stats and a level 18 artifact vs an 810 dps with terrible stats and a level 3 artifact? Should this tank still be doing 50% of the damage as this dps? Where is the bar we set here?
2) Fight duration. Trash dies quickly. Tanks can often get a few GCDs off more than dps can, just because they set the pace and start fights. Without tanks needing rampup time, this will heavily skew quick fights. At what length of the fight are we saying tank should be x% of dpsers, and at how many adds?
3) Scaling. How much dps will tanks do in ilvl 900 gear vs dps? Will they still be competitive? At what level/ilvl do we need to start seeing tanks doing x% of dps? 110 level with 810 ilvl?
4) SKill. At what percentile of play should tanks be doing x%dps of what percentile dps?