“Not getting old” does not mean “not having offspring”, competition of slightly mutated offspring is what evolution is driven by.

And the expectation is that no ageing and no disease will give us a 50% chance of surviving any given (IIRC) 1000-ish year period, because accidents will still happen.

In nature, you still have predators and lots of diseases that limit lifespan even of those species which have biological immortality: https://en.wikipedia.org/wiki/Biological_immortality