I understand why they do that, since it's supposed to be an encouragement. But there are two aspects of the Nobel prize: reputation and prize money.

If they would change the rules to make exceptions for shared discoveries, it won't dilute the reputation boost for the scientists in question in the slightest, and if they give the money to the surviving winners that problem is solved too.

Yes, I know that it’s not awarded posthumously.

My understanding is that she was not given credit in her lifetime so she would not have been awarded it anyway. Unless, someone had explained her contribution in those 4 years.

“Although her works on coal and viruses were appreciated in her lifetime, her contributions to the discovery of the structure of DNA were largely recognised posthumously.”

There's actually an apology from - Watson, I think - as an addendum in my old copy of The Double Helix bought in the 1980s.