I think the woman OP was describing simply is not aware that she is monopolising conversations and taking up people's time with things that are of no importance to them.
Those who live alone tend to talk far too long to anyone who can be expected to listen.
In my opinion, the receptionist should, when she noticed OP waiting, have said to the woman engrossing her attention, "I am sorry, but there is another patient wating for my attention, so I shall have to ask you to leave now."
This behaviour has nothing to do with a sense of entitlement, whatever that is, I have noticed all sorts of bad behaviour being ascribed to it these days.
Anyone who behaves inconsiderately, should be politely requested to show more consideration for others.
Well done OP for mentioning it to her.