It was a 100% deus-ex-machina moment, just hardcore leveraging a character who had never previously been in the show or even really hinted at, for the nostalgia equivalent of a shotgun to the face. If it wasn't Luke, the reaction wouldn't have been tears of joy but rather would have been "What a cheap deus-ex-machina ending that was!".
So what, you wanted them to have Han Solo or Leia or someone equally nostalgia-y turn up in a similarly pure-nostalgia moment? Maybe instead of the cruiser crashing, the Millennium Falcon could have inexplicably hurtled into the base and Chewbacca could have leapt off a ramp whilst it hovered and started whaling on Moff Gideon?
I mean, that's what you're praising. Brutal, unsubtle use of nostalgia.