As to the genetics of an old doe, here is my take:
For "survival of the fittest" and genetic reshuffling, you don't want a stagnant breeding pool. Once a doe has contributed her genes to the herd a couple of breeding season, she has placed her genetics into the herd and I want things to progress from there. Old does might result in "line breeding" where closely related individuals mate. Line breeding can reinforce some desirable traits in a small portion of the offspring, but the other portion can get all of the undesirable traits. Having her contribute her same genes for many seasons could minimize the "hybrid" effect, where hybrids show greater vigor
As to an old doe who has survived many season being one who is smart and thus should be allowed to pass her genes on, well, maybe she is buck toothed, wears a plaid dress with thick glasses and hangs out at the library. Her only redeeming trait would be that by hiding at the library she survives (her longevity). Do you want to reinforce only one trait?
Me, I try to shoot as many as I can, trying my best to not shoot buttons. I've shot mom before, then shot the yearling that hung around.
Shooting a doe with twins ended my wife's desire to hunt. She clipped the femoral artery of the doe with her broadhead and then came to get me to help trail. The blood trail was easy to follow and when we got to the doe, we had to chase her twin fawns off. My wife was not impressed to say the least! So I can sympathize with the position some have taken about not wanting to shoot fawns or does with fawns.