It seems that songwriter contracts will have to be re-drafted to reflect the new paradigm. If the accountants can figure out how to pay the publisher in this new system, one would assume that a mechanism could be found whereby the songwriters will be paid as well.
If they can't, then the songwriters will go on strike and the artists will suddenly have a very limited pool of songs from which to perform.
Songwriters sell the rights to their songs to publishing companies because they individually dont have the cash to do the non-stop, random, deep forensic audits neccessary to discourage labels from hiding sales and shortchanging the writer(s). If legal action against a label is ever neccessary on a songwriters behalf, the publisher has to eat the expenses, its part of the publishing deal.
A big songwriter can establish their own phantom publishing company and the big publishers will administer the rights on their behalf, which slightly ups the percentage of the (rounded) 9 cent-per song per-unit sold mechanical royalty.