I was taught in a different context that dealing damage is just mitigating future damage received. Killing an enemy a round sooner means you've denied them a turn's worth of attacks against the PCs. So yes, that's effectively the same thing as denying them a turn's worth of attacks now. But the question is, what did it cost you to apply that effect, and how completely did it lock them down for that turn?
As I said (nearly a month ago), it's the trade that matters, because action economy is king. Getting three for one, where your one turn locks down a powerful enemy for three turns, is the best case scenario. If they make their save immediately, or use Legendary Resistance, it's less of a good trade. If the target is weak enough that you could have just as easily killed it with the same number of actions, it's a complete waste.
The reason that Monk's Stunning Strike is so good is that it's a free rider to your normal attacks. All the cost is shifted to Future You when you run out of Focus Points. But if that never happens it's pure upside, and in the here and now it costs nothing to your action economy. Now imagine you had to trade an attack to use Stunning Strike. Suddenly it's a lot less good, isn't it? That's what I meant by having to trade damage for utility.
