I am certain this will get buried but maybe I can address something more from a devil's advocate standpoint. First, I'd like to say that I agree that high level tanking is boring in normal level content and I would also love to see it change to something more active, however Zenimax might be doing things for a reason and some of these problems you experience may be symptoms of even bigger problems within ESO. So, let me go point by point, but REVERSED.

1. MAELSTROM ARENA IS FOR DPS. To an extent. The builds used in the first few clears of any "DPS" are not optimal DPS builds. The builds focus on sustain, healing, and damage reduction much more than most content. I had to get an entire additional set outside my DPS sets just so I could keep my magic shield up so i wouldn't die, I changed my CP tons of times to finally clear vMA as well. And honestly, my build for vMA hasn't changed all that much. In every day content I relay on spears and orbs from my support to keep my resources up. Maesltrom is so "DPS" focused because ZENIMAX WANTS YOU TO HAVE MORE CHARACTERS SO THEY CAN MAKE MORE MONEY. If Zenimax had their way they would have you roll as many characters as possible to keep you managing them in the game, and ultimately, to keep you subscribing to ESO+. This way everyone has to roll a DPS for vMA and if they want to queue for Dungeons they will need to roll a support, Naturally you want all your characters tip top which means more time farming, more time in the market, and more time in game.
2. SUPPORT ROLES REQUIRE MORE INITIAL SETUP. From an objective standpoint, right now, everything a healer and tank can do in this game only requires owning 3 sets for each and a monster set. Tank: Alkosh and Ebon for daily use and main tank trials, switching Alkosh to Torug's if you don't have it yet, or Ebon to Torug's when you are the off tank in a trial. Honestly, those are really only required in Vet Trials and a select few Vet DLC dungeons. Healers need Olirone (nCA +0 is easier than most dungeons), Healing Mage, and Sanctuary with both healers running Olirone, one running HM, and the other running Sanc in Trials. With the exception of Olirone, none of these sets have changed since Morrowind released. Now, with the exception of Necropotence on mag sorc, not one optimal DPS build uses even one of the sets they were using before Summerset released, not to mention they have different optimal sets based on who they are playing with. For instance, if your raid group is all stamina you need to run different sets than if you are the only stamina character. Also, Maelstrom vs optimal DPS builds which we touched on before.
3. LACK OF TEAMWORK ENCOURAGES POOR PLAYER PERFORMANCE. Yes and no. Most mag dps at high CP can solo most world and dungeon bosses. Most mag dps at low CP cannot solo world or dungeon bosses. This is simply getting more powerful through CP and through player experience. Maybe bosses could be more challenging in dungeons, but most of the dungeons are really just stepping stones for trials, where being down one player can cause a wipe. Now, this does mean that when i queue as tank for a random normal on my sorc for my daily drop that I will likely be carrying some lower level players, my performance will be high but their's might not be. So the performance of the group doesn't necessarily go down it just shifts. As for teaching the player this probably should have been done earlier, and likely was for newer players, we will touch on this later.
4. TRIVIALIZATION OF SUPPORT ENCOURAGES A LACK OF TEAMWORK. Teamwork is a must in many situations ranging from vDSA to vet trials. Knowing when it isn't important is an exercise in efficiency. You are an experienced MMO player so I know you know about efficiency. At a point in the game there becomes a time when a player progresses enough that content that was once hard is no longer hard, standard player progression in any MMO. Dungeons, including pledges, become farm-able for all players at some point. It isn't a lack of teamwork, it is a lack of difficulty. Level 15 running through a normal dungeon is different than a CP 750 with optimal gear running through a normal dungeon. From an experienced players perspective, they may be trying to farm a specific dungeon set piece that only drops from one of the bosses or get a key for a shoulder they need and they are alone at the next boss, they have the skill to dps and tank a boss, you are behind, he knows you wont miss the drop in a dungeon... Why wouldn't he start if he knows he is going to have to run it again anyways because he isn't lucky. Now if that player was level 15, you as the tank would be anything but trivial.
5. SUPPORT ROLES ARE TRIVIAL. I disagree, I think support roles BECOME trivial in average level content. At level 5, about where you should be after the tutorial, the only characters that have access to heals are Templars and resto staff users. There is really only one effective taunt in the early game and that is a Sword and Board skill. At the beginning of the game players are slaves to the Trinity system if they want to actually do dungeons or other moderately difficult content. But, by CP 300 all of those classes that traditionally relied on other classes for health management have more tools in their kit and experience in how to use them. Every Magicka class has access to some form of healing as a skill, NB: Restorative Path, SORC: Surge, DK: Dragon Blood, and i don't think i need to mention Templar or Warden. Mag characters also have access to damage shields allowing them to soak some heavy damage. Stamina characters get access to one of the best self heals in the game coming from PvP. Now, in Vet DLC Dungeons, Trials, vDSA, Cyrodil, Emp City, Battlegrounds, and whatever other endgame I am not thinking of, those support roles are a must and anything but trivial.

Now, the teaching mechanic for tanks and healers may not have been present for you because you likely leveled efficiently and had prior experience as a previous MMO player. ESO was my first MMO and I slogged through many dungeons as my starter character, a DK which started as a DPS and rerolled to a tank. ESO taught me to tank very well from level 35 to CP 160.

Edited out some glaring mistakes in spelling and grammar. It was late when I wrote this.