Interestingly, the community manager mentioned that while yes, losing a tank is bad, that applies to any situation. Losing 1 player will always be bad, such as losing a support means less utility or healing for your tank and DPS to do their jobs, or losing a DPS can mean less pressure which enables the enemy to do more.
And also tanks weren’t always to first to die, because having two healers on one tank is a huge sustain buff versus having two healers try to keep rein and dva healed.
And I think some people forget that in some ways, removing one tank actually lowers damage because for one thing, you have one less tank to do damage. Compare a Reinhardt Zarya comp to just a Reinhardt. No Zarya beam on your frontline, no Zarya on an off-angle trying to pressure out support, so less damage.
Also, one less tank means your team doesn’t get enabled as much to do damage. Back to the rein zarya example, in current 6v6 a reinhardt will get bubbled which enables him to walk in and swing without getting punished. In 5v5, the zarya wont bubble him, causing him to do less. Or similarly, no shield or beefy 500 hp rein with armor causes more focus to go to zarya which forces her to play more slowly and do less damage. It also applies to DPS, such as in a normal dva comp, you wont have the dva to give resources to the DPS like matrix or body blocking to enable more positioning.
At the end of the day tho, it’s all speculation. We have a fraction of an inkling of an idea what it’ll be like until we actually try it. I’m optimistic for 5v5 and the effect it’ll have on the current problems that plague the game