The core issue is about making tanking feel valuable. People are not going to invest time into a role if they aren't going to be valued for it.

If your role has a low impact, it doesn't matter if you're performing at the 'ceiling' or the 'floor'. There's no difference in value for your team between bringing a good player or a bad one. This is why relative dps matters.

This is also why having a skill differential matters. The biggest thing that inspires people to invest time into mastering a job is seeing a top tier player perform at a high level and seeing the impact that it has. That's why we go to watch star athletes make big plays. The problem is that this has been sequentially neutered at the altar of "accessibility". If everyone is a winner, everyone pays their subs. Or so they think.

But if everyone can achieve it, it's not worth achieving.