That's the system working exactly how it was designed to.
Riot can't be sure if a client crashes, if someone hit alt-f4, or if someone went into task manager to kill the process.
It sucks but the system has to judge all cases the same. It's also the same to your teammates. 7-8 min down one team member is a pretty big disadvantage that they couldn't do anything about. That disadvantage will be there regardless of the reason the person disconnected. So if a team wins despite that disadvantage that's a testimate to the 4 members skill more than the person that disconnected.
If you choose to remain disconnected Leavebuster is likely to judge you more heavily since then you're gone for an entire game rather than just ~25% of one. There are more punishments than just an automatic loss on the match that the Leavebuster can dole out.