Your would be ruining 4 other ppls game due rustiness meta map changes etc
The same could be said for a lot of factors really. Maybe you're on tilt, maybe you're just playing badly that day for no particularly reason, maybe you're stuck on a hero you're extremely uncomfortable with somehow.
While I understand the idea and advantages by an MMR reset, or "seasons", if you will, I don't think it's necessary. Sure, sometimes a bad player can ruin a game, but it happens less often than you think. Many games with a bad player/intentional feeder/whatever are totally winnable. Yes, it sucks that you got saddled with that on your team, and I get that it's frustrating.
In the specific example of someone that hasn't played in a while and might be rusty to all the changes, help them out. Tell them "Oh you should get Aghs on AM, he got an Aghs last year and it's really good" other things like that. I guarantee you that even a rusty player will still have some idea how to play the game, and is totally different than a new player.
The MMR system is fine as it is for the most part. It's not perfect, but it is still a good measure of skill. You play better, your MMR will improve.
Personally I just need to play more solo games. I don't play them that often.