First we really need the Source code of this tool though if we really want this tool to stay for posterity!
"If I make something so great then people will still pay me even if I give it away for free!"
A tiny percentage of people might still pay for it, but the vast majority will not, and will complain loudly about any attempts to monetize.
This is true of all products though, in ecommerce 1% conversion is pretty average... generally the path is to increase the top of the funnel because it's always small at the bottom... some products get by with 0.5% or less
You said it yourself, this is a hobby FOSS developer. Key word hobby.
Have people stopped understanding that others do things simply because they want to? Not everything is profit-driven, even on this site.
Though I can also understand the child comment on your comment as well. I think that is a reasonable statement as well.
This is a really weird ethical dilemma that I still hold, as a hobby creator myself (just not at a scale of really earning money,but boy I sure would want to ), I want to earn money from it but I also want to distribute it freely, I am not sure how I can get both.
What is the cultural significance of this tool that would require it?