Spoiler:
2-tanks are dangerous they have inner utility which is not accessible to melee and better durability and are cooler....yes they also are ppl will prefer use a big /shiny armored char than tiny little bw; tought this also open problems in what is what if tanks can take the place of dd then ppl will use all tank and no more DD.
3-some sm proposal failed exatly due to sm being able to go full off in tactics and hit like a truck while remain durable as other tanks due the fact it have limit support in other field and because KOBS exist is fine as it is.
4-because it have other hard to balance utility such CC and more group oriented morales with have a greater impact so you both take 1 char with no real downside exept it need a guard and have no cc immunity so basically 4x tank new meta could be a thing if it's not done properly; the exemple is a moba; 1 = 1 with some little difference but mmorpg is different story. And if you do not want to se the rvr roam by only tanks it's better be catious.
these are my answers and im neutral on the matter.