The issue isn't tank damage. It's the relative damage that tanks do in comparison to dps.

In ARR, there was a fair amount of overlap between some of the better WARs and some of the weaker BRDs on account of STR accessories, even amongst skilled players. In Stormblood, you would see a bit of overlap, but only if you compared the extremes of skill. Right now, it's difficult to do less damage than a tank as dps unless you're actively trying to play badly. This is a reasonable change to an extent, because you don't want groups substituting in tanks for dps to make mechanics easier.

But the problem is that dps is the only thing that has an impact. And it becomes harder to see good tanking or good healing as things that carry value. The vast majority of boss fights are on rails and they position themselves for you, so you aren't really going to notice movement decisions that your tank makes. You'll die if you don't mitigate a tankbuster, but there's never a question of not having the gear to meet a mitigation or healing check. DPS have always been a higher priority for gear, but at least previously you could justify throwing a few early gear pieces to your tanks or healers geared up relatively quickly if they were shouldering a bigger chunk of the dps burden (and if tank gear happened to drop anyways, well, great for them). Between the new gear system and the relative dps gap between the roles, it's pretty hard to justify getting any gear to your tanks and healers before all your dps are geared up, irrespective of skill.

You don't need to incentivize people to play dps. Yes, they make up the majority of your subs. But if players switch roles, queue times go up and subs drop. The biggest reasons why someone picks healer or tank are to have an impact on their team and provide value to their teammates. You achieve this by making these roles more challenging in order to give them value, rather than the reverse. The devs need to take a long hard look at how they design fights from a tanking and healing perspective, and make these roles feel like they can contribute something useful again.