I think the issue has less to do with punishing the rage quitters (i can, to an extent, accept the current punishment escalation), and more to do with how it affects those who are playing the game in the correct spirit as it was intended. The real problem as i see it, which Riot doesn't seem to care enough to fix, is that those who do not rage quit from the game are also punished in the form of a full LP loss anyway (excluding the irrelevantly low number of games that are won even with a rage quitter).
Riot implemented a protection from players who dont load at the start, but ignores the players who suffer from a later than 3min quitter. If Riot actually gave a shit, in my opinion, they would use some form of protection in ranked such as a reduced LP loss in the event of a rage quitter or post 3min leaver. This would still allow them to punish (as they currently see fit) the rage quitter or leaver, while partially protecting the players who enter games in the spirit they were intended.
Its been many years though, and by all available evidence, Riot appears to give very few fucks for those who play in the correct intended spirit of the game (the vast majority of their players). But i ask, why would Riot make an effort to take care of and protect their players who are the victims of the rage quitters (instead of only punishing the wrong doer)? Clearly their priorities lay with simply releasing new skins occasionally, making a quick buck, and acting as though all is well.
Only a few more weeks until WoW Classic can deliver us.