The coins aren't really the issue. Eventually people move on from needing Castrum even if the coins are only there, and participation dies off. The real problem is "you need to sit in a given instance for Castrum to pop then you need a lot of people in that instance to want to do Castrum" to actually be able to do it. That will not work when participation drops off because most players are done with it, because why would I ever go back there now that I'm done with it?
It's a nonsensical design decision in a game where they already solved this problem for everything else with Duty Finder and roulettes. The problem was entirely predictable and SE walked right into it anyway.