That would pivot a lot of those merch and drop-shipping hawkers on YouTube to “here’s how to make your own FOSS library and collect your friends’ €100!”

We’d get a lot more of “things that qualify for the €100”, but it’s not clear that we’d get a lot more useful FOSS.

Your average citizen probably doesn't know what open source is, let alone could name one project.

So most likely most people would not give anything because they don't know/care, and a few marketable projects (like VLC) would get most of the rest, way more than they need.

Not all science is taxed directly by governents based on politician's whims.

FOSS could be funded by global (non-national) consortium

Normal people don't care about FOSS or even know what it is.

They would probably use that 100 on food or fuel.