The same people who edit the article can decide
who gets credit. When you edit an article you can decide for yourself
how much credit you deserve, and if some subsequent editor disagrees,
they can change it, or discuss it on the talk page as with the the
contents of the articles.
We get enough heated debates about content, can you imagine trying to
reach a consensus on whether or not a particular user's work is worth
crediting on the page? Pretty much everyone involved in the debate
will have a conflict of interest!