So suppose everyone (who wanted it so) were right now made immortal. Except, for moral reasons, the possibility of suicide were left open.

How confident are you that human beings would be around forever? Or for, say, a trillion years?

Right now, I don't want to die. There's more I want to see and do! And even if I can't think of anything, I think I'll come up with something new.

But after a trillion years of subjective experience? I really don't know. What kind of a person can keep themselves ticking for that long without just getting bored to death? Finding something for us all to do that will occupy us forever is a non-trivial existential problem, assuming most or all of the other ones get solved.