What logic makes plants better to kill than cows, better than octopi, better than cats? because vegans might the argument uncomfortable doesn't render it invalid either
It's also entirely implausible, from an evolutionary perspective, that plants should have developed the capacity to feel pain on a scale similar to other organisms given that they haven't developed the ability to move away from situations that might cause them pain (pain being a proxy for survival risk).
Keep in mind that instead of fight or flight plants have a range of internal chemical responses available to minimise damage and that this does make pain plausible:
Videos of plants calcium signalling response to damage:
You can find a lot more material in this vein.
My point is that we are “animalists” and not very well predisposed to recognising the true nature of plants given how different they are to us.
You instinctively know what fear and pain are, and minimising those things is the point of veganism.
If plants feel their fear and pain far slower, then much less of it is happening.