It cannot be another Lost. The Severance mystery box has an anti-plutocracy, humanist foundation. It would be really hard for them to not make it satisfying.

It's trivial for writers to not be able to live up to the expectations of a huge number of dedicated fans.

I agree, but I think that's a higher bar than "not being satisfying", which is what Lost was (putting it very generously). Fans often get carried away with mystique of mystery boxes and anything short of an orgasmic existential experience would be a letdown, but that doesn't mean in an absolute sense it would be an unsatisfying or bad end.