I've realised there's a model that works at that scale which I love, and it's the one being pursued by Continuum Analytics for Numba vs NumbaPro: the "delayed open source" model.
What this model does is help establish the credibility of the company as a centre of engineering excellence in their own right (and thus worth investing in, either directly or as a customer), while still providing the risk management benefits for employees and customers: if senior management at the company lose the plot and start taking actions that are customer or employee hostile, then any business risk only applies to the last 6 or 12 months of features and employees that decide to strike out on their own are starting from a point that isn't too far behind.