This is the real answer. Nature only cares that the old already mixed their genes. So, from an evolutionary biology perspective, it's good to clear out the old stock at the end of every generation. Nature already got whatever it was gonna get out of us at that point. To allow that set of genes to stick around actually yields negative returns, ie - less chance of producing improvements, at that point. (Again, just from the perspective of "Mother Nature" so to speak. Obviously the animals themselves would rather not die.)
Mother nature has no perspective. There are no improvements either, only things that are and are not. Everything is dependent on local context at the present point in time and framing whether it is "good" or "bad" (e.g. one could frame our intelligence as good or bad given how it resulted in anthropocentric climate change that may doom us in time, while 'dumber' species have existed for millions of years without wiping themselves out).
What works today may not work tomorrow. What didn't work yesterday may work today. Evolution is not linear. It tends to happen in bursts almost like a dam busting loose. A constant cycling between periods of genetic diversity (i.e. big resetting just happened, no winners taking all quite yet), and what is known as allele fixation (a winner emerged, oh god no...).
>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.
