[The_Reader_David]

But this is another evolutionary "just-so story": if an ecological niche were occupied by 'immortal unless slain' creatures, evolutionary biologists would devise another story to show how the survival strategy of perserving individual organisms and replacing them only at a slow rate was vastly superior in the face of the survial pressures faced in that ecological niche to competing survival strategies of more prolific reproduction and short life-spans.

I am, perhaps, being a little too scornful in calling it a "just-so story". I call it this because it is claimed to be a "prediction" of a general theory--which I would argue it is not precisely because evolutionary biologists would behave as I have indicated. In fact, the account given is a proper scientific theory in its own right, which can be tested and falsified (by finding a population of slowly reproducing 'immortal unless slain' organisms), modified to account for the contrary data (provided the new modification is still testable) and so forth.

[Junior]

if an ecological niche were occupied by 'immortal unless slain' creatures,

Some sponges fall into this category, and evolutionary theory covers them, too.