The fraud stops with getting through security and bypassing secondary screening.

Bottom line is that all airlines reconcile boarding passes with reservations in order to get an accurate passenger manifest. So even if they tear it off and wish you well, but it is reconciled before the plane leaves. Ever noticed a gate agent pick up a stack of stubs from the boarding agent??? And a seat dupe will be investigated as well.

The only loophole is if you know someone with a reservation that is going to no show for a flight and you create a bogus boarding pass, encode the barcode with their PNR, and use up their ticket. But you may as well have just used a fake ID to check in as that person.