Thanks to the relative ease of implementing patches in multiplayer games, most people are familiar with the concept of rotating "metas," at least for games that do regular balancing updates. Perhaps upon a game's release, certain weapons or characters are stupidly powerful, which causes everyone to gravitate toward them, and then one update later, they become incredibly weak and a new set of stupidly powerful weapons or characters becomes the most popular thing ever. Of course, the idea behind regular balancing updates is to make it so that everything in a multiplayer game performs equally well, but that is rather hard to do when there are so many variables at play. Nowhere else is this more apparent than in Overwatch, where (with more than 20 different Heroes) any singular buff or nerf can disrupt the game's established meta.
Lately, the multi-tank meta in Overwatch has caught a lot of attention, with some calling it wildly broken and overpowered while others merely conform to or ignore the meta with varying degrees of success. Whether or not this relatively new meta is overpowered is up to you, but the reasoning behind why such a meta is becoming more and more prevalent is rather simple. At its core, Overwatch is a game of distance, with victory being determined by your team's ability to advance even just a couple of meters, and there is no "easier," and retrospectively obvious, way to do so other than by having a team composed of Heroes that are designed to mitigate damage. Not factoring in things like abilities and armor into the equation, tank Heroes like D.Va and Reinhardt have somewhere around three times the health that most other Heroes have, which is a necessity given their role as damage sponges. This means that a DPS Hero like Soldier: 76, who does 20 damage per bullet, would have to empty his weapon's entire magazine into Reinhardt, who has 500 health, without missing to kill him (this is not factoring things like armor, abilities, optimal range, etc.), which takes some three seconds to do.
Throw in all the abilities and whatnot, and that three second time-to-kill increases exponentially, especially when Ana can do at least 200 points of healing within a couple of seconds, if even that. That most of Overwatch's tanks are quite capable of dealing damage on their own, albeit at close range, only further aggravates this problem, since you now have a scenario whereby the most obvious counter to all of these tanks are other tanks, given their high survivability and natural ability to initiate pushes, which is a necessary part of their role. Obviously, in an actual game, there are many more variables present and there are countless numbers of finer points that you can bring up, but the underlying theory behind the multi-tank team composition is still solid: the longer that it takes for the other team to kill someone on your team, the easier it is for you and your team to advance and or kill them.
To (seemingly) counter this influx of multi-tank team comps, Blizzard has unveiled a few tentative changes to a couple of Overwatch's more popular tanks, D.Va and Roadhog. In short, D.Va will do less damage and take more damage, while Roadhog's ability to instantly displace an enemy and kill them is (supposedly) much harder to use. On top of that, Ana should be able to contribute less healing, which theoretically sounds like it would neuter the multi-tank strategy (although in practice, she can still heal a disgusting amount of damage in no time at all, which is what enabled the current meta in the first place). Whether or not these changes are enough remains to be seen, but only one thing is certain: the mere idea of having a team full of tanks has already been unleashed and thoroughly tested by Overwatch's audience, and most people aren't going to forget how seemingly effective it was any time soon. Swing the nerf hammer too far, though, and Blizzard might create even more problems, including but not limited to potentially making DPS Heroes the new overwhelmingly deciding factor in the game.
There are certain alternatives that exist to nerfing everything, however, and it mostly revolves around the lack of any Heroes or abilities that can debuff the other team. Abilities like Zenyatta's Orb of Discord, Sombra's Hack and EMP, and Ana's Biotic Grenade can make it easier to kill even the healthiest of tanks, but given that Zenyatta's Discord Orb is the only consistent ability that can really make a difference when fighting masses of tanks, it is almost a non-factor unless your team can capitalize on efficient orb placement. Perhaps a redesign of certain abilities and Heroes is in order (a la Symmetra), giving some Heroes (maybe even a new Hero) the ability to debuff armor, health, and or damage (which could make a big impact on how aggressive tanks can behave) and other Heroes the ability to cleanse said debuffs. In any case, it is, as ever, up to Blizzard to find a good middle ground between keeping people's favorite Heroes viable and changing the meta to the point that the previous one no longer dominates every match.