I agree with you that business and forst class cabins are separated for a reason and have more number of toilets per pax than in economy. The challenge here is, how do you tell an economy pax not to cross the curtain as they have not paid to be seated in that cabin area? It is a very uncomfortable situation, where cc have to find a solution that affends no-one. In an airline I used to work at, when there were long queues for economy toilets we used to discreetly "escort" one economy pax at a time to the business class toliets. I don't think anyone in business class ever noticed or had a problem with that.
At the end of the day, everyone has the right to use a toilet when one is available, rgardless of what seating they paid for. And the answer I think lies exactly here, economy pax have not paid to SIT in business cabins and enjoy the difference in service there but they have certainly paid to be able to use toilet facilities in whichever cabin this may be available. As long as they don't disturb the "piece and quiet" biz or first pax paid extra for, I don't see how they could be denied access to toilets there.