I don’t think anyone intends public funds to be quite that sticky; public education is itself a public good, and having once attended a public school really has nothing to do with developing a product 20 years down the road.

Also, writing open source code can support a viable business. Not every example has been successful, and some have been sold to hypercapitalist owners who wanted to extract more profit, others have failed to keep up, but Canonical is doing alright with it, Red Hat did for a long time, among others. Plenty of bigger tech companies also employ people to write open source software, despite it not being the company’s main business, React, PyTorch, TensorFlow, and so many other projects. Those engineers definitely aren’t working for free.