> People living longer = fewer births assuming stable population

That's a big assumption, and one that seems rather unlikely to hold or to suddenly change.

> You'd literally be trading a teenager's first crush for an 80-year old's third marriage.

If you're going to blatantly invoke imagery like that, then how about preserving the wisdom of experience? How much more might we advance the state of the art in every field if the experts in those fields are still around, healthy, and making new discoveries?

And there are plenty of 80-year-olds still enjoying their first marriage.

Youth is a wonder largely because of aging; eliminate aging and you get the pleasures of youth when you're a hundred.

Personally, I'm satisfied with "death is bad; stopping death would be good". Simplistic yet still incredibly true.