Again I have said nerfing tanks healing is a good thing I don't know how much more I can mention this. But again like you've acknowledged it's more then sustain, I would personally rather they start by toning down the tanks kit entirely, then looking into things such as fight design and healer kits
Fights should never allow for two players to wittle down a boss slowly (unless it was already at low percent), This is done by both reducing sustain and mitigation values first but also making sure item levels are properly synched and fights actually do enough damage.
Though I can understand your view point I think as Tanks and Healers we should stand together and become the Supportstrike command force and become more powerful then ever leaving dps to solo content... Ok maybe off track here but we should be both asking for content to make both roles feel good and important as largely we've always been a after thought as Non-dps, hence why this awful imbalance between the roles has only festered into what we have today, I think nothing more then a overhaul of how both roles work is the only acceptable solution.
Though if in the meantime that means remove tank sustain, so be it I may not wanna play the game because I no longer feel like a "paladin" but if the healers would finally be happy then I guess theirs no greater sacrifice, but I'll still keep complaining until I actually feel like a "Paladin" until the ends of time.
In what world would it be slower? as long as you know when to mitigate for AOE's in bosses and don't make mistakes It wouldn't be any slower... You really are overestimating tank sustain at this point and underestimating tank mitigation, fight output ect. I've even managed to do stormblood dungeons without no tank or healers and no rdm/smn this games damage output is a joke, though admittedly if you go no tank you cant wall to wall