Imo, the best argument against suicide is the fact, that someone is very likely going to be devastated by your decision and I believe that's true for the vast majority of people here. Someone, maybe a mother, a father, a brother, a sister, a grandparent or a good friend of yours will likely be devastated by your decision and I don't know how to argue against that.
I think suicide is ultimately your choice and if you have suffered for many years or even decades and without any hope for it to get any better in the future, then suicide is certainly not a "bad" idea or immoral, but... It will probably still hurt someone a lot for a long time and I really don't know what to say about that...