Basically , if you ( the species ) live forever then how would expect the natural selection occurs?
This underpins
That you have reach the pinnacle and no can adapt in any kind of environment
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