> I can't speak for the idea that menopause exists because it is useful for old women nurture their grandchildren

Exactly. The most we can say is "it's generally useful so it tends to happen". However, evolutionary biologists tend to paint their explanations with a certainty that just isn't there, and can look at any old random thing and say "well, this is obviously because X!".

"Evolution made spiders eat their mates because it makes sense that they would want to preserve energy". Yes, but it also makes sense that the mates would stick around to feed the offspring, otherwise all species would kill the males after mating. We aren't as sure about why things are the way they are as evobio would have us believe, and not to mention evobio explanations for human psychology, which are usually completely ludicrous.

In general, the process for explaining things using evolutionary biology seems to be:

* Look at behaviour.

* Find one advantage, however small, of said behaviour, regardless of disadvantages.

* Say "well obviously this behaviour evolved like this because of that advantage".

* Publish.