I don't disagree that the healing plays a part in how strong the mitigation is both play a part into making a system where tanks are invincible even my listed examples of reducing tank's kits I said I would have entirely removed all healing from the magic blades combo as I stand by that, I don't think we need to remove Holy sheltrons healing if we keep other aspects of sustain on other tanks.
Guardian shouldn't be a 2minute cooldown it should just be weaker, adding barriers to cooldowns like suggested would open for more opportunity to cycle it so you still take barely any damage, instead of self healing now you just use your barrier + mitigation cooldown for high damage instances, I'd assume if you wanted to add barriers to more tank defensives you'd likely go for short cooldowns like holysheltron and HOC but having them up so often would lead to them never taking much damage in the first place
Funny you bring Dark knight up because the fact is 1 tank and 3 DPS runs are still EASILY done with dark knight, I know I can do a run as Dark knight healerless, people don't mention it enough because they see big unga bunga warrior and think "well OBVIOUSLY its the self healing that's the problem" when for the entire time im practically screaming and shouting that self healing isn't the only issue and If you played tank you would know that it isn't a simple self healing issue. (Though again as i've said a thousand of times yes it plays a big part into the issue)
Lastly even if we removed all self healing and I mean all self healing from everyjob for the sake of healers, even without turning things into barriers, you would still BARELY need to heal maybe throwing one ogcd heal once every two minutes is exhilarating gameplay for you and what exactly you want from healer, It's certainly not what I want we need to tackle, The Mitigation, the Self healing, the encounters, how item level synchs if you want a proper system that actually works for all roles.
We will not get anywhere better if we only look at one aspect of the problem and ignore the rest, Removing self healing doesn't solve the issue unless your issue is "how dare other jobs have some healing cooldowns I want to throw my one ogcd heal and my one aoe heal every few minutes so I have fun" How about we actually design a team game where healers are actually healing tanks that take actual damage.
I'll lastly say this again I do not want to play immortal mitigation simulator as a tank I want to play a Support role who protects my team.