Last survivors quitting can also help enemy teams get more RP by pushing up their placement. For example, you may have friends in enemy teams, so you can avoid some fights to let your squad mates die, then avoid picking up their banners so that you remain solo. At this point, you can quit and push your friends one place up if they're still in the game, while also taking out of the match the RP that another enemy would get by killing you. Considering this perspective, getting a leaver penalty makes sense even if your chances of surviving alone are slim.
I'd say that players should get some form of punishment from killing themselves as well, like a few RP if they jump off the map or die in the lava or outside the ring without an enemy damaging them in the previous 20-30 seconds. Or maybe fixed costs, -10 RP for falling off the map, -1 RP per tick when outside the ring or walking on lava.