What do you propose happens? Riot magically makes people see the true meaning of christmas so they never leave games? They put bots in instead so that people can get fed in what is still basically a 4v5?
You can't blindly give loss prevention for 4v5s, because that is extremely abusable. There is no way to ultimitely fix the fact that yes, sometimes life happens and people need to leave games. Riot should do something about games that are 4v5 from the start, but that is a small enough minority that it's not worth much time, and then creates awkward interaction around when those players actually come back. And even then, a system where 1 person doesn't show up could easily be abused; a duo goes into games, one person never shows up. The other gets elo for every time they win a 4v5 (which actually happens about 8% of the time) and loses no elo ever. They slowly but inevitably climb the rankings.
This is a game, and it will often be unfair. Maybe you'll end up against a diamond smurf and lose, or maybe your top lane will feed 20 kills to a hungry cho'gath. That's just how it goes. If these types of things make the game so it isn't fun for you anymore, don't play. But please stop complaining about complicated issues unless you can offer a feisable solution.
