Minor point but attending a service is not nothing. It is time that could be far better spent appreciating the art and architecture of the building. I do not know for certain but I strongly suspect that if you started wandering round the abbey during a service you would be asked to sit down or leave.

As to the difference between Westminster abbey and cathedral, yes,the age is significant but is not the cost in maintaining both very high? If so, why does one charge and the other not? Anything to do with one giving something back to the people while the other, being part of the state, is still taxing them? Just asking.