All of this is true, but sometimes people forget that buying a solution doesn't mean there will be no work. Sometimes, the integration and maintenance of that integration ends up being more than building and maintaining your own solution, especially if you have to do any large changes to how your systems work in order to integrate.
In unreal and unity, most of the third party code I’ve tried to use ended up unusable because of either bugs, performance issues, or limitations. There are great exceptions but it’s been frustrating overall. The ones that do work, well, the documentation is often terrible and it takes longer to learn and integrate than it would to make your own. Finally, unity has a history of retroactively changing their terms of se…
For example make a JSON serialization library and you'll be asked "how do I spawn enemies from JSON" which is arguably entirely orthogonal to your library but don't answer and get downvoted for bad support. Do answer and you'll just be asked more questions about how to adapt your example to their personal project and you'll have to teach them programming in the process.
These incentives make it almost impossible to make money selling 3rd party code (plugins/add-ons). If you charge what it should really cost given the amount of work put in the market won't bare it. If you charge what the market will bare you'll go broke.
Maybe Unity should split the market into "Pro" and "Hobbyist" and the Pro market would have prices more inline with what it actually costs. Check out the prices of libraries like Radgametools.com (you'll have to google the prices) for comparison.