The case against legalized suicide is the same as the case against legalized murder. Sure, there are some outlying times when murder is the right thing to do. Making it legal is still a bad idea because it changes the decision calculus from moral to economic, which has been empirically shown to increase immoral behavior (ref. Drive by Daniel Pink). If somebody needs social approval for murder or suicide enough that they would fail to do it otherwise, we judge that they are not, on average, in a desperate enough position necessary to morally justify their decision.
We also judge that the motivation of the antinatalists pushing this political program is to flatten decisions of life and death into the singular economic dimension as part of the globalist Enlightenment project to exterminate most of the human race. It’s like the Gnostic story where the UFO takes the pneumatics away from the dirty human animals to live in utopia, except in this story the pneumatics make everyone else go away.