Journal publishers worry that university libraries will stop paying for journals. This would take away the funding that currently supports (a) profits and (b) a quality control process for papers via peer review.
The current unofficial system differs from the official system. In the official system, an interested reader must be affiliated with a paying institution to get access to papers, and thus institutions have an incentive to pay. In the unofficial system, everyone has access to published papers, and university libraries pay to maintain the status quo.
People generally agree that it would be nice to bring the official and unofficial systems together, but nobody knows exactly how the new system will work.