I'm unfamiliar, are they a business or just a platform? As much as I'd prefer it all be open, the only way to send a message is to work in their sphere. Contracts between publishers and libraries exist in the conference rooms, decided by committees. No librarian or administrator will be convinced without face time a salesman who can convince them to dump their past decades of loyalty. How can a librarian, one trained in maintaining and preserving access to materials, trust that a new and free platform will be around for years? The publishing houses have been around for centuries, literally.
Somebody needs to come along and start by making the little, obvious things right. Their next goal is to avoid acquisition long enough to make a difference. Every so often a promising business comes along, only to be bought out by the big guys and slowly dismantled (see SerialsSolutions).
Also, science and math journals are only an aspect of journal publishing, albeit the most pricey of subjects of which to build a collection. These subjects have always been the first to lean towards open access but they stand in distinction from the rest of a library's collection.
PLoS is a non-profit publisher. All of their journals are open-access and are paid for by publication fees. They have only been around for a few years, but in that time, their top-line journals (PLoS Biology, Comp Bio, Medicine, Genetics, etc...) have become top-tier class journals (one notch below Science, Nature and Cell, depending on who you talk to).
They are no lightweight in the biomedical research journal landscape.
The publishing houses have been around for centuries, literally.
One of the interesting things about scientific publishing is that the current arrangement is a recent phenomenon. Many journals started out as independent or university-affiliated publications which charged to cover costs; then were gradually spun off or bought out by for-profit institutions through the 60s and 70s. The accretion of publishers into a small number of companies, each with a virtual monopoly on critical journals in the field, resulted in the insane balance of power we currently experience. Inelastic demand for information (it's extremely difficult to do research without access to the major journals in your field) means Elsevier et al can jack up prices almost arbitrarily.
PLoS is free to access, but not free to publish, it's quite well established, and financially doing fine. There is no reason to assume they will just shut down any time sooner than the ancient paywall-powered publishing houses. And, believe it or not, the people behind PLoS actually care about open science, so it's not likely they will sell off either.
Besides, it's not just PLoS , there's Frontiers (frontiersin.org) and many other open access publishers. In fact many paywall-journals allow authors to make their papers open access by paying an additional fee.
