>What works today may not work tomorrow. What didn't work yesterday may work today.

All the more reason for variety. Which stops happening when only one set of genes is mixing. (Assuming that set of eternal genes would have any motivation to mix at all.) If you want a higher chance at producing the genes most resilient in a given context, aging and death are a necessary part of that process if you're stuck with sexual reproduction. You need motivation, and you need maximum mixing.