The co-operative model seems to me a very good compromise between a no-profit foundation and a full-fledged for-profit company. Very, very interesting proposal.
I would favour this model over the two alternatives. It combines the “benevolent” aims of a foundation, with the useful bits of a private company (resource allocation attraction of funding, etc.).
I believe this difference is important, because one who may send 100€ as a donation to a foundation, may be willing to put 5000€ as an investment in a company or in this case, a co-op.
However, in order to assuage any concerns by the Nextcloud leadership, I think we should consider “golden shares” or alternative ways so that the co-operative does not prevent effective leadership of the project.
This is potentially one flagship project for the FLOSS community, and I think it would be good to take time to reflect and not to fear experimenting with alternative ways of governance.
In my opinion, leading by assembly does not work for day to day decisions.