What you are getting into here is gov law/policies and how govs earn/spend money. Yes there are millions of possibilities and alternatives on how governments can run their countries.
What I am simply telling you is that Thatcher's statement is mathematically impossible. The money is always there in any economy and someone always has some, therefor you can never run out of other people's money.
You are correct in a fiat currency economy if the gov is run by morons who create too much debt and too much money, the money will loose value and the currency will collapse, it has happened in many countries throughout history. But that has nothing to do with Thatcher's false statement.
I think you are reading more into that statement than what the quote meant.
Sure there is other people's money. I believe what she was talking about is that you run out of other people's money that you can GET AT. Sure, it accumulates other places. Yet if that is beyond their taxation methods then it is gone as far as the socialism is concerned.
Well I have no idea what she was thinking when she made her statement, but that is my point, the money is always somewhere, other people always have money.
How a government chooses to get that money or if they want to create new money or if we want to live in a society where the few hold the majority of the currency while others cant afford to eat, that is another topic...
Basically it is the difference between a one line quote, and an essay to explain it.