This was a fundamental question for me before taking the plunge into starting a coop. Long story short, I did a lot of research and there wasn’t anything that really compensated risk in early stage startups hence the dearth of platform coops.
We ended up structuring our coop to have equity split from voting rights to allow employees to have ESOPs and investors to invest as they do in traditional corporation minus their control of the board. In theory we would be able to IPO down the line, and perhaps become the first coop to do so without demutualizing or a separate investment vehicle on the side.