Unless you are SCH, there are very few pulls a reasonable healer cannot keep a tank alive through.
I have seen plenty of pulls (in all three post-legacy patch cycles so far) where I can literally be spamming Cure 2 as fast as I can (as in, stopping to try to do anything else like a Regen would result in the tank dying) and it's not enough to keep up with the sheer damage the tank is taking.

It is not just a healer effort.