You paid to board in group X rather than group Y (for some value of X < Y). You got that, although it sounds like group X and group Y were boarded together. Wo

You paid to board in group X rather than group Y (for some value of X < Y). You got that, although it sounds like group X and group Y were boarded together. Would you be complaining if the entire flight was full of people in group Z (for some value of Z < X)? Or if you'd bought a day-of-travel upgrade when you would've gotten a complimentary upgrade with your frequent-flyer status? Or if you'd paid extra to select a seat at booking when the seat you wanted turned out to be open at check-in time? Or if you paid to change your flight rather than wait on free same-day stand-by? For all you, or the checkin agent, know there could've been irregular ops that moved the entire passenger complement from another flight to yours after you checked in. Unlikely, but it happens. If you're really that worried about, write the airline and complain. They may give you a refund or some free miles or something. However, you got exactly what you paid for and entered willing into the transaction. And, to be clear, literally the entire business of airlines (which is yield management) is based on asymmetry of knowledge between between travelers and carriers.