OP, I don't think you need to be rude to stop this cyle, when they say how skint they are just say 'I know, can you imagine how much we feel the pinch given our earnings' and leave it at that. They'd have to have a pretty thick skin to continue the conversation.
If they do continue it, word your thoughts more strongly, their thick skin can obviuosly take it.
Doing no more than moan about it on here isn't going to change anything.
I do think they are being unreasonable but unless you do something about it what was the point in the thread? Did you just want to be told they ABU and be happy you were right?
I also think you like exaggerating both ends of the scale a bit for effect though.
I still think it is absolute bollocks that BIL will earn £900k straight out of training, even if he is a barrister, but you have chosen to accept that as gospel.
Likewise, you said you work 'long hours' and work full time, whilst I understand why you work the hours you do, they are neither long, nor full time.
And as I and others have said, it does not take 6 years of full time education to improve your skills to allow you to earn a higher salary. A higher salary does not have to be 'a wonderfully high paid career' just more than you currently earn.