Surely you don't expect someonoe that uses a particular piece of software only during work to donate to that from their own salary.

Developers can always contribute as well but the benefits aren't really accruing to them. If open source libraries didn't exist there would be commercial offerings or things would have to be built in house. Also, $100 / year / dev is a rounding error and would add to a company's engineering brand.

It seems like a win / win to me but, of course, no one has to do anything at all. Just don't get upset when maintainers pull the rug out from under you because that is the behavior that is being incentivized: 1) make your thing widely used and hard to replace, 2) get in to large companies that can afford to pay 3) change license, pull rug and get your legal team ready to fight.

One need to see how many developers are already getting paid enough and out of them how many actually donating to OSS projects.

AFAIK developers are full of excuses like "these trillion dollar companies need to pay fair share while my hundred thousand dollar salary in this big expensive city leaves me with nothing to donate.