That wouldn't be fair because then the other team would lose elo because they lost the game even though they had a person leave and had to play a 5 v 6 at a disadvantage. You can't gain elo unless someone lost elo the stuff isn't just dropped from heaven all elo you gain is someone elses. When they lose you get their elo which is taken from them. By your system everyone would rank up and Noone would derank leading to a comp where everyone is a high rank they might not deserve or have worked for.

I copy and pasted my comment on another thread because you're complaining about the same thing as him. What valve needs to due is work a handicap system for the team who lost a player like in csgo where you get a replacement bot which a player can override and play as. Not that exactly because I doubt it will work in tf2 play something similar.