After traveling through Europe and seeing the amount of "pay per use" bathrooms, I am torn.

On one hand, there are many more public washrooms than in North America, and they are generally much cleaner. On the other hand, paying to basically be a human being feels wrong and I refused to use them.

I've used the toilet on a plane maybe 2 times in my life, and I've fly quite a bit, so this probably wouldn't impact me much at all.

Usually you will only find those in germanic culture countries, I never got why. I mean the ones inside services like coffees, restaurants,....

There are the single purpose ones in the middle of nowhere or train stations, where payment is an attempt against vandalism.

Everywhere else they tend to be free, and contrary to being on a plane, one can search for an alternative.

That would run into the FAA; there are a number of things that happen on flights that are mandated by the FAA, I suspect restrooms (and the drinks) are some of them for health reasons.

Ryanair did it >10 years ago, coin slot not contact payment terminal however.

They didn't actually do it. The Ryanair CEO, Michael O'Leary, has a well-practiced approach of making outlandish claims about Ryanair's thrifty innovations that echo around the internet; discount publicity. About the same time he claimed that a standing-only section would be introduced for £1 tickets. Of course this also didn't happen.

Not in the flights I regularly take.