I think you underestimate the value some plugins have. I won’t speak for anyone else, so I’ll speak for myself with Nexus Unit Testing. You don’t need a plugin to easily run automated tests you write, nor do you need to write tests, but it really pays for itself. The up-front cost (which in this case is free if you get the source code) and time to write tests covers itself many times over later one when you are re-testing or testing rare conditions. It especially pays for itself when it points out a problem with your data saving code that would lead to data loss during an outage. It has easily helped enable tens of thousands of dollars in revenue for just me.
Not all plugins should be paid. For now, there is probably going to be a mix of deserved and garbage plugins up for a while until the actually worth-it ones rise to the top. Paid plugins will give people an incentive to make better plugins, as well as compete with others at a better price. Just give it time, and realize free plugins are not going away.