I don't know if that's true. "Readers" currently means "research libraries", who act on behalf of the scientists doing the research, so shift isn't so great.
Additionally, open access journals still must maintain an image of prestige and quality -- authors want to publish in the most prestigious possible journal. There is incentive for journals to compete on quality even when paid by authors.
In any case, I think the role of journals as gatekeepers for quality is overstated (peer reviewers and editors generally do a bad job assessing article quality), and as science shifts to alternative forms of peer review and publication, journals will matter less.
Additionally, open access journals still must maintain an image of prestige and quality -- authors want to publish in the most prestigious possible journal. There is incentive for journals to compete on quality even when paid by authors.
In any case, I think the role of journals as gatekeepers for quality is overstated (peer reviewers and editors generally do a bad job assessing article quality), and as science shifts to alternative forms of peer review and publication, journals will matter less.