Your example is about ongoing damage or damage reduction buffs against an opponent with the same HP and damage profile as a PC in a one on one duel. None of which are common circumstances in a D&D encounter.
Let's look at your example, but instead of ongoing +1 damage or +1 damage reduction, let's look at a 1d10 damage boost or a 1d10 heal in a fight you'll lose if you don't use it. With the heal you have to wait until you're almost dead, use it, and hope it buys you the extra turn or two needed to win. You'll probably win, but it takes a long time and you take a lot of damage. If you use it as a damage boost, you can win on the first turn without taking any damage, and freeing your future actions to go support your other party members.
That's the power of burst damage. It kills targets faster, which means you take less damage and gain more advantage in the action economy. Unless you're in a long boss fight, and I mean long, in equal amounts burst damage is always better than combat healing. For combat healing to be a thing it either has to be way cheaper or you need a dedicated healer whose damage is lower value. Which is not the paradigm that D&D works with.