How about an argument for 50%, based on symmetry:
There are two special seats: yours (call it Y), and the one that was assigned to the first passenger to board (call it F). If any passenger after the first happens to sit in F before Y is occupied, you will get to sit in Y. But if anyone grabs Y before F is occupied, you will not get to sit in Y. As there is nothing to make either of these more likely than the other, the chances are equal.
Does that work?