Are there lives not worth living?

This is a question/​clarification post.

I’ve been reading some bits about population ethics recently, and I’ve seen many people assume that there are lives not worth living, with examples usually involving a tremendous amount of incurable suffering.

However, I don’t see why this should be a sure thing, so I would be very happy to hear your best arguments for it.

The opposite claim would be that any life is better than no life at all, which I don’t necessarily find intuitive, but does not seem implausible either. After all, under a utilitarian framework, setting the zero-utility level seems relatively arbitrary.

The claim that all lives are worth living is often associated with arguments against abortion or euthanasia, but opposition to abortion does not necessarily follow from the claim. It could still be better that a person does not exist if that person’s existence negatively affects others.

So, my question really is about the value of one life taken in isolation: could we imagine that a suffering existence always has a positive value, even if a very small one?

Thanks for sharing your thoughts on this. Links discussing this issue more in-depth are also welcome.

PS: I’m new to the philosophy of effective altruism, and I’m not a philosopher myself, so I’m sorry if this is a silly question (and I strongly suspect it is).

PPS: This is my first post here, so please tell me if there is anything I should be doing to follow community standards.