There's nothing stopping you from charging money for FOSS software. Sell the first one for as much as it cost to write it.
If you found one willing buyer while it was still closed source, odds are there are others. Why deprive yourself of additional income? Open sourcing does happen from closed source, but usually that's long after the income stream has exhausted itself (see, e.g., https://github.com/microsoft/ms-dos).
I'd argue the best possible model for goods that have no marginal costs (aka IP) is the "pay me to create it" model.
As in, let a creator decide how much they value creating the work. Then pay them that to create it - then release it free for everyone.
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No ridiculous societal restraints that cost time and money to enforce (poorly) as IP law. Compensation is happening. Creating is encouraged.
Practical constraints make it unlikely we'll get much closer to this than Kickstarter (and equivalents) but it's a very honest model, that works with intellectual property by default.
Developer presents the idea and how much it costs. If enough people pitch in, then the idea is funded, the developer gets the money as a lump sum and can turn the idea into reality.
Why did this model fail?