We can't say that it was a mistake because it is not even our Bitcoin and it's not our business to decide what to do with the Bitcoin of other people.
It might be a waste of opportunity to others, but maybe that someone has a valid reason why he did spend his Bitcoin to pay a house rent, (1) For experience, (2) maybe he just don't have any money on that period of time. For me, as long as you spend it on the things you need, it will never be a waste.
exactly what we can say for what do the person used Bitcoin to buy pizza back in the days , if we are questioning OP's decision then we must question also all of those personalities that uses big funds just to promote bitcoin in the days when people are still denying its existence .
people must be proud of everything about bitcoin and if given a chance I will also use my bitcoin to buy or rent a house but suddenly this is not available to my country .
Bitcoin is a digital currency just like fiat to be used as payment for service and others, Bitcoin was created as an alternative to fiat, so there is absolutely nothing wrong in paying rent in bitcoin, one thing should be certain there should be an agreement between the sender and the recipient, and also an understanding of the dynamics of bitcoin so they do not think its fraud, that too should be put into consideration.
I don't know to others but I am good using Bitcoin to pay everything under the sun , this is how I trust Bitcoin and this market.