Chaosmancer
Legend
While you are correct, you are also wrong.
You need not undo all damage to be an effective healer. Actually it is very frustrating if healing in general outperformes damage dealing. This will just make fights very unsatisfying.
What you need to do is heal at the right time. Sometimes healing 10 damage is enough to keep your tank from going down from the next 30 damage.
The poison condition might be irrelevant to the tank.
AKA Whack-a-Mole.
That's literally WHY the whack-a-mole problem exists, because you are just buying one more round. But if your tank has taken 10 damage, then healing them 10 damage AT ANY POINT could keep them going down from the next 30 damage... because health is static unless damage or healing takes place.
The problem is that if you are t 50/60 hp, healing doesn't feel worth the action or spell slot. You aren't in danger. But if you are at 8/60 and the enemy can easily hit for 21 damage, healing 10 is pointless, it isn't enough. So you let them take that 20+ damage, then heal them next turn, preventing a chunk of damage.
Now, you are sort of correct about too much healing being frustrating, but there are limits on this. For example, being able to stop a single monsters turn isn't going to drag on the fight if there is one healer and multiple monsters. And if you have a party full of dedicated healers... then yes, out-healing the damage is their strategy and it should be viable to attempt. And if they get frustrated with it... well, they are the ones implementing the strategy.