Nerfing tanks will break the content where all their mitigation and healing is actually needed to survive (Ultimates, week 1 Savage, and stuff like M6S adds phase).

If you don’t want them to be able to solo casual content when the party is dead then the encounter needs to have enough damage that the tanks can’t handle it by themselves for more than a minute. Or have some lighter “bodycheck” mechanics that do extra damage if you don’t have enough people alive to do them.

M3 Normal is a good example of an encounter that tanks can’t deal with all by themselves. The tankbusters and raidwides do non-trivial amounts of damage, and the towers do even more damage when they’re not all soaked. I’ve had a few runs where if the healers die halfway though it then the party just has to wipe and retry.

The consequence of requiring your healer to stay alive would mean that you’re basically guaranteed to wipe if your healer dies to a boss that isn’t almost dead. This wouldn’t be that common in 8 player content since that requires both healers to die at the same time, but you’ll probably get walled for a while by certain dungeon bosses if your healer is a first timer and is dying a bunch. The occasional dungeon run where the tanks and dps beat a boss without a healer will now become a dungeon run where you do a boss over and over until the healer learns it.