I agree, my thought is less that I think an amoeba feels pain (I don't) but that I don't personally believe things with insect-level complexity feel pain. That they react to negative stimuli definitely doesn't prove sentience, and I personally find the totality of how insects interact with the world to be too extremely mechanistic to believe they feel at all. This is true of crabs as well, I've never really looked into lobsters enough to have an opinion on them.