As you can no doubt guess, I am betting that Users will be mainly using the free version. If there is still a market for the paid version, that's fine, but irrelevant to the goal of providing the entire corpus online for free. Of course if there is a free Archive, a parallel fee-based one with added enhancements is perfectly fine, if there is a market for it. In order to appear in refereed form in the free archive, a paper needs to have been refereed and accepted by a journal. For if it is not paid in any way, the quality control and journal system collapses.
Authors draw on the publication funds to pay their page charges up-front.
The customer for that service is the author and, more specifically, his institution, for quality-controlled publications are the measures of research productivity on which their funding is in turn largely based. Referees actually do the quality control for free, but implementing it costs a little money, and that's what the page charges are to pay for. The market decides whether it is worth paying for publishers' proprietary add-on enhancements. Peer-review itself needs to be financed in any case. You also suggest that it is desirable if many existing journals, esp.