Anti-speciesism and compassion are at the core of vegan philosophy. Vegans have historically extended compassion from only other humans, to animals so different from us as insects and fish, and this inclusiveness of compassion, not simply a different form of line-drawing, has been its major virtue.

Denying compassion to plants, the major component of any vegan's diet, is irrational and incongruous. It shows an insecurity in the vegan morality and a willingness to draw the same line in a different place so as to feel morally justified in eating anything at all. This is line-drawing is identical speciesism. Any organism can be conceived as an automaton and categorically denied compassion, but vegans have chosen not to do so, and that is at the heart of the philosophy.

I can understand when people deny that plants feel pain, even if I think it is wrong. However, I believe that predation on plants is not likely to be as painful as predation on animals because of the ability of plants as decentralised organisms to regrow tissue, whereas removing an animal's forelimb, for example, is a permanent disfiguration that the animal will have to carry for the rest of their life.

Ziq gives an argument here that we can sustain ourselves without killing plants, at all.

Veganism also takes up far less land than carnism, thus, in addition, far less plant life to sustain us. This is not a new argument.

That aside, the no-sentience(/no-pain) argument does nothing at all to realistically address some valid (but misused and generally hypocritical and not addressing the point in context) arguments by carnists, namely that predation actually is a major part of how ecosystems function.

My understanding is this. Usually, organisms make more offspring than can survive. That's natural selection, right? It is the way species survive despite their hardships, by excessive fertility. So a species often ends up filling most of its carrying capacity in the ecosystem.

That has two implications. One is that if every one of these creatures lived and thrived, things would be overly crowded, and the ecosystem would not function well. Consider rabbits in Australia. This is the one that is cited specifically to justify hunting. The second is that if creatures fill up most of their carrying capacity, they should not consume more than they need, because if they do, it undermines their future food security. As humans we can keep this in mind, and try to predate as little as possible on other beings. Yet we can still recognise that with no heterotrophy at all, things would be out of balance, and that would result in greater suffering.

TLDR: Potatoes are sentient beings deserving of compassion, but thinning them is better for them and for us than either eating cows or starving to death.