You have to get so many things just right for immortality to 'work', i.e. to actually be useful in a survival context. It does no good for creatures to not age if they lose a significant amount of viability.
But real immortality in the complete reversal of aging, I think would be an immense advantage. Instead of having to teach your young over a very short period of time the tricks they need to be viable adults, creatures could super-specialize. One creature could supply all the food needed for a huge colony. They'd just get better and better at exploiting their niche and other creatures could find other niches.
I think if intelligence hadn't 'gotten there first', immortal creatures would be running the show.
We call this a "single point of failure".
In the case of a supposed aging gene, consider that genes themselves preferentially want to exist in more fit organisms. If a gene could somehow manage this (i.e. be embodied with more fit genes), it will itself spread. But this is exactly what an aging gene does! Consider how an advantageous gene spreads through a population. This spread is a function of the number of generations that have passed. So a gene that could speed up the passing of generations will increase its own likelihood of existing in more fit organisms (increasing its own fitness), as advantageous genes within the population spread faster and are more frequently "paired up" with the aging gene. But the subpopulation with the aging gene is more fit than the subpopulation without it since the time for advantageous genes to spread is decreased, and so the aging population overtakes its counterpart.
What you're missing is that 'good' is defined with respect to a goal. From an evolutionary standpoint (whatever that really is), the complete extinction of all life is just one possible way things could be and it is as good as any other way things could be. From a human perspective, this is awful. As is any future where humans don't exist.
