I agree that it's not at all a silly question. And I also agree that more imagination is required by some on here . . .
First, the OP seems (to me) to be positing the notion of "feeling" you are over a particular place, as opposed to the point where that specific feeling dissipates -- which it indeed does. If one is flogging around at 5000 feet one sees the details that make it obvious where one is: houses, specific rivers, mountains, etc. There is a point where it all melds into "one earth" below and, to me at least (and perhaps to the OP), sovereign states become less recognisable -- and more irrelevant.
Second (to HD), despite your somewhat flippant "where else could you possibly be?" comment, one is NOT over the UK (for example) above a certain point. As a previous poster noted, countries have so far been unable to agree on (and are perhaps reticent to pursue) the notion of "upper limits" of territorial airspace. Clearly, there comes a point (one hundred miles, one thousand miles, twenty thousand miles?) when the concept of being in any country's territory is both absurd and unenforceable. The "where else could you be" is "elsewhere" or "space".
grizz