Life: it's better than the alternative

But is it though? How can we know. It's not just comparing apples to oranges, it's comparing apples to unicorns.

When it comes to apples and oranges, it's possible to have a preference. You can experience both of them and use your senses and preferences to make a judgement. How can you do that with a unicorn? You'll never encounter one because they don't exist. We can define unicorns in whatever way we want (because they're not real) so we can simply say "unicorns are better than apples" and, for whatever hypothetical scenario or work of fiction we're talking about, it's true. But we can also define unicorns as worse than apples and it's equally true. Because a unicorn is whatever we say it is (because they don't exist).

Well, how can we compare existence with non-existence, a state we can never experience? A person who doesn't exist can't wish they did exist, because existence is a prerequisite of wishing. If you say "I would prefer not to exist" then you're talking nonsense, because a person who doesn't exist has no preferences.

When we say "I [would] prefer strawberry ice-cream to chocolate" we are either comparing our memories of experiencing those flavours or imagining what that experience will be like. We can't remember not existing and we can't imagine not existing, because no matter what you imagine, you're there. Existing. Preferring non-existence is definitionally impossible. Which means, so is preferring existence. It is simply impossible to rate the two states comparatively.