> But lenders won't lend without a high expected ROI.
Exactly, that's what's stopping them. Worker cooperatives don't provide the return on investment that investor ownership grants. They don't allow those with capital (wealth) to demand a large portion of the returns from the business. That's what requires the new economy - removing the option for capital to demand ownership and control so that it has to make do with a reasonable, regulated return on its loans.
We don't use worker cooperatives in the same way we don't use corporations. There is no overarching entity that uses any of these things.
If worker cooperatives are so great, they should be outpacing corporations. Some do. For example, companies like Winco and Mondragon exceed productivity of their corporate counterparts. That's fine. More power to them. Their story should encourage others to start worker cooperatives.
For example, there's the form of Democratic Socialism I propose above: where we replace the current business structure with worker cooperatives funded by regulated loans or crowd sourcing that compete with each other in a free market. We know worker cooperatives work, because there are many successful worker cooperatives operating well in the current capitalist economy. There's no reason to believe they would stop functioning well in a free market economy where all businesses are worker run.
That's just one example. The next system project has collected a number of additional ones: https://thenextsystem.org/systems
And like I said, I have a whole bookshelf full of alternative proposals. Some of which don't make a whole lot of sense. Some of which are very plausible. And some of which are already working in micro, just waiting to be tried in macro.