I dunno, maybe it happens a TON in your games, but in my ranked games I'd say it happens maybe 1 in every 10 games at the most. Given that it happens as often on the other team and I usually get a free win from it I figure it balances out. If we're ahead a decent amount or they clearly can't team fight well and we can win 4v5s and maybe take the game you stick it out. Otherwise you can just surrender and move on to the next game, no big deal. You report them after the game and if it's a chronic problem they get banned, if it's just their internet dc because of a storm or something they get a warning and it isn't an issue because they don't do it a whole lot.
Maybe it's just me but I don't think it's a big issue. Sucks to lose ranked due to it, but it's nice to get free wins when your team is losing lanes and you win due to dc. I always apologize to the other team when that happens, but it is the balance for when it happens to you and it is nice to get the LP back. It's a wash if you play enough games though so honestly I don't think riot needs to do anything. Now if you have DC/AFK in 4 out of 5 of your games and only have a 5v4 game (or 4v4 since your rate of dc is so high on your team) where the enemy is disadvantaged every 10 or 15 games, then the issue isn't DCs because your games differ statistically from the norm. If you are getting significantly more DCs and AFKs than the enemy team over 100 games there is probably something in your behavior that is encouraging team to pick your game as the 1 game they just leave in every 10, 20, or 100 games they play. In that case the issue isn't what to do to leavers, but what to do to you.