Concede is still no fix to ragequitting.
Think about it, why is ragequitting a problem. It's a problem because 1)it's impossible to win 3v2 or 4v2 etc. games if you play against equally skilled/better players and 2)AI actually helps enemy team by giving them gold/exp.
Both problems can be solved by giving buffs or advantages to the other team with less players or making the AI much better(even if it involves making the AI cheat) and killing AI would not give exp/gold to the enemy team. Suggestion like this have been made here many times.
Just because somebody ragequits, it should not mean either play and lose after 15min or concede and get concede stat. Sure, the concede option should still exist incase players dont like playing outnumbered even with the advantages they get because somebody left. But players should also have the option to continue playing and have a fair chance of winning, even if outnumbered.