There has never been a "we" or "us" in Team. There is however a "me" if you mix the letters up a bit.
I understand your dilemma and feel for your condition but I do not condone your solution in any way shape or form.
Forcing groups to "stop" progression, completely and utterly "stop", until they have obtained a member of a specific class is game breaking.
It's all well and fine when the first wave of players hits the content and smashes right on through it, but when the rest of the players (mainly DPS) come through unable to progress because they do not have a healer or tank? Seriously?
Do you really think they are going to just sit there and wait and draw straws for who gets to go with the next healer to come through that area again?
I wouldn't. I would expect them to say enough is enough and quit a game that was designed with such a shortsighted flaw...
Resulting in your game getting shut down because the majority is no longer paying your server bill.
Well, the title of this thread, "Where's the I in Team" implies that you think MMOs in general or ESO in particular are Team based games, when that isn't necessarily true. ESO in particular for the first 150 levels (level 50, plus 10 VRs that are roughly equal to ten regular levels each) is 95%+ a soloable experience. It isn't until you hit Craglorn that some (all?) quests actually require you to have more than one person.
So I was pointing out the fallacy of your original statement, that MMO and/or ESO does not mean team based game. If you want a team based game, I suggest baseball, football (American or the other), basketball, etc...
It's almost gotten to the point now where people don't know how to group. If the game had been like that at launch it would've appealed to an entirely different sort of gamer than the playerbase now. Expecting the current population that has soloed the vast majority of the game and has not learned the mechanics by doing it for 100+ levels to suddenly learn and do it well is laughable. You'd just have a bunch of people who suck and who you wouldn't want to group with anyways, because they have little experience doing so.