The trinity is a playermade concept from the dawn of the genre. In the beginning, there was no taunts and there was no idea about the trinity. There only was aggro mechanics, since mobs were supposed to attack someone. These aggro mechanics were used to strategise the gameplay by players to develop a group roles where one took all the aggro and kept it to enable others to deal the damage and then they had a player devoted to keep other players in full health. Thus came to life the idea about tanks, healers and damage dealers. This was eventually embraced by the developers, the community and the industry and it became a golden standard.

Now, you could argue that there is a fourth role - a person who buffs other players and debuffs the enemies - a buffbot or a support, but this is not the case in ESO. Our community made the support role unneccessary by creating healer and tank builds that have support role's base covered.

How this is relevant to the thread? Well, I'd like to argue there is no 'no role', there is no fourth role in ESO. You can find out your role by asking 2 questions:
1. Do you take the aggro and keep it? (if yes: you're the tank. If no, move on to the second question)
2. Do you heal other players? (if yes: you're a healer. If no, you're a DD)

More survivability doesn't make you a tank and doesn't make you not a dd, since it doesn't change the aggro mechanics and doesn't taunt the mobs. It just makes you a dd with more survivability, be it more resistance or a higher health pool. Having self-heals doesn't neccessarily make you a healer either, it could if your self-heals also healed others.