That's a generalisation and a half.
I think it's fair to say that most of us don't expect to be competing with an equivalently competent dps in damage output, as entertaining as it was in ARR/early Heavensward.
At the same time, when most content tends to utilise one tank at any given time, that second tank is functionally expected to be a part time dps. That needs to feel rewarding. Otherwise it just feels like you're sitting on the bench waiting for your turn to provoke and take damage again. And your sense of 'reward' is always going to be based on what the dps player next to you is doing. At the very least, you should be able to nip at their heels.
What I would personally like to see is Healers -> Tanks -> DPS (in ascending order of damage output), with either a small gap or a small amount of overlap between the top and bottom of each cluster. Looking back at previous expansions, there's definitely been an improvement on how damage output has been balanced within any given role. But the progressive downward shift in tank damage output relative to both dps and healers is not good.
What's the magic number for the difference? It's a subjective experience. Start incrementally tuning us back towards where we were relatively and let's find out. But a starting point to this would be to see a commitment from the dev team towards making tanking more fulfilling again. We are not just here to "take damage".
Relative damage is what matters. You can increase all three roles by the same amount, then increase boss HP proportionally, and there's no net change.
Your perception of what is a "strong" attack is always going to be contingent on what the rest of your team is capable of doing.
The majority of the marketing around tanks in the lead in to this expansion was on their new damage dealing moves. So much of our new move budgets go into Fell Cleave-esque upgrades, because that sells expansions. How very impressive and powerful they are! And all is well and great and until you look at the healer shooting sparkles next to them.