No, people really don’t get this. Okay, scenario:
You’re playing Reinhardt. Your shield is low so you drop it to let enemies chip at your armor while you get heals and you start launching fire strikes. Ana hits you with anti and starts firing at you, negating the Mercy/Moira duo that is backing you up (for now).
Up to this point that enemy Sigma and Hanzo have been abusing your shield and trying to take space. The enemy Baptiste and Soldier: 76 have been doing other things, mostly out of position where they could also abuse your shield.
You aren’t near enough to cover where you can get out of trouble, so you raise your battered shield to ride out anti. Mercy flits away to damage boost your Sojourn, while Moira starts leeching health from the enemy Sigma.
Everyone on the enemy team realizes you’re purpled and behind a weak shield, so the entire enemy team prepares to focus you down. Ana, Hanzo, and Sigma are already firing on you. Soldier and Baptiste are ~2s away from repositioning to fire on you. Your team’s dps isn’t doing so great, so your Sojourn and Cassidy aren’t getting any meaningful picks over the next 4s and are mostly just shooting the Sigma, who has just put up his barrier.
What happens next is heavily-dependent on how long your shield has to last and how long your supports have to wait to go back to healing you.
Nade currently lasts 3.5s. In this example, the combo of Sigma + Hanzo (assume Hanzo has already used Storm Arrow to weaken your shield) + Ana can pull 73.3 + 97.4 + 93.75 dps (assume Ana doesn’t have to reload in this situation). At around 264 dps, they can bring down your half-health shield in under three seconds on their own. With help from Soldier and Baptiste after 2s have passed, they can have it down in about 2.5s or less.
That would give the enemy team about 1s of antiheal remaining to peel off your armor and then kill you. And give that enemy team comp, they certainly would be capable of doing the job.
Take 0.5s off the nade duration and they only get 0.5s to hit you without incoming heals. That gives you a chance.
Take 1s off Nade duration and nade becomes useless. By the time your shield finally breaks, nade has already worn off.
Reducing heal % changes the character of biotic grenade entirely. I can still save you with Transcendence. Ana can use her own nade to offset the heal penalty and heal you like normal (not currently possible). Etc.