All of this is interesting. I am curious what the current license means for interacting plugins. My sister is an IP lawyer, I'll ask her what she thinks. It will probably not be for a couple weeks.
I think one of the main unanswered questions is what would premium plugins do to the ecosystem. Would there be more or fewer plugins available? Would users have more choice? Would there be more development? Would the quality improve? If this option limits development, quality, and/or user choice, the answer is obvious.
Also, no built in OctoPrint support would be necessary, of course, but then each developer would need to write their own system. That doesn't really seem to match the goal of increasing the amount of development. Think of the various 'app stores' out there. For a fee, developers could place their plugin in the store, and a portion of the money could (and should) go to @Foosel, or whoever maintains the store. This provides an additional marketing benefit, since the majority of premium apps would be in one place, which would encourage users to search.
Also, I was thinking about plugins I would pay for, and the first one I would have purchased would be the enclosure plugin, and perhaps the gcode system commands plugin. I like themeify too, and would consider paying something for that. Since I started development of my plugin, I've not had a chance to try many other plugins, so my experience is probably a bit unusual. There are likely many more out there.
Also, if no source is available, I would think long and hard about making a purchase. If source is available, why purchase anything? I've not yet been able to resolve this conflict in my mind. Perhaps extra features/convenience? Or maybe a sort of pay-what-you-like model? Not sure. If there were a 'do you want to tip the developer' popup or something, could that be a good alternative?