The answer seems pretty straightforward:

Supply and demand steps in and the value of the remaining BTC goes up accordingly.

As the BTC pool reduces the value of a single BTC goes up and you get more purchasing power for that single BTC. What was 1BTC yesterday is 0.99 tomorrow and 0.90 next year, etc.

You could potentially keep this going down to the point that a world supply of 100,000 BTC would place a value of 0.004761904762 BTC on something that previously cost 1 BTC, etc.

Of course, if we are down to 100,000BTC from 21M I suppose the community would devise a new currency standard - at which point your remaining BTC would buy that new currency at market rate.