Do you tell others to think about how happy you could be in a few years when they are sad? No, you don't.
Do you encourage systematically poor people to suck it up because things will get better?
Same mentality is used to justify racial injustices and inequality. Things will get better in future so suck it up for the movement. These people are in pain and should be able to stop it if they see it not worth it. Obviously there can be some time period such as a year or two before you can access euthanasia. Otherwise, it's cruel to let people suffer for years in false hopes.