The problem is you are literally consigning people to unnecessarily painful, lingering deaths now, because of some change that you think might possibly happen in the future. Also, if you are talking about physically fit people suffering from depression asking to be euthanised, I shouldn't really have to point out that unfortunately such people do already take matters into their own hands. A quick search shows that in 2022, 5642 people committed suicide in the UK. I would guess that a significant proportion of them were suffering from some form of depression and that improving mental healthcare provision would be of more use to them, than blocking a law that would give a dignified exit to those in the greatest need who are not in a position to resolve the situation on their own.