you could sexp the Indart to die
That would be pretty cheap. Not a great suggestion cause it would just infuriate people. Having more raiders jump in is the better solution.
I disagree on that. It should be made clear to the player that there was nothing they could do. They may carry on trying to save it otherwise.
Put in a message to go with the scripted death, making it clear. Something storyline, like the stress of the battle caused some kind of death (reactor overload, hull breach, whatever.)
Even breaking the fourth wall and straight out telling the player that the ship has to die, but well done for killing everything then blowing it up would be preferable to me than trickery with new waves of fighters that only spawn if the player wipes everything out. Something like that could even be a fun easter egg type thing in a game.
There's also potentially still a risk. I remember seeing one video of FS1, where some player was sat on a spawn point with his wingmen. As soon as the waves of fighters for the mission were jumping in they were obliterated before they could barely even move. I'm talking like 5 seconds later everything's dead.