That's never going to work.
First, people AFK for 2 reasons:
- Internet issues / IRL stuff / etc... which is what you're complaining about
- Because they're losing / rage quitting
In case 2, your suggestion is just brutal. Oh, so since I stomped my lane and got up 4-0 in the first 7 minutes and made my opponent rage quit I don't deserve my win LP? That's BS. I wasn't handed a win - I MADE him AFK. I earned that win.
The second problem is that people would be trying to encourage allies to AFK in order to save their own LP. Oh we're down 15-5 at 17 minutes, [role] you're the feeder (say he's 1/5/2) so you need to AFK and take the hit. He refuses then gets abused and the other 4 gang up and threaten to report him. It would make a pretty toxic environment.
The third reason it won't happen:
Riot is of the belief that it will more than balance out for you. If you NEVER AFK, then there's a 4/9 chance that the AFK is on your team and a 5/9 chance it's on the enemy team. Which means that you'll on average benefit from other people disconnecting. Which means overall that people who AFK will slide down the ladder and you'll move up. Which is a desirable thing (having AFK'ers move down the ladder).
Their belief is based on the law of large numbers, i.e. if you play enough games you'll realize it. So while it may not work out for an individual (so far this season I've had 2 ranked games with an afk [who admittedly reconnected both times] and 0 ranked games with an enemy DC, and also 3 AFKs in normals but only 1 on enemy team) but it will work out if you play enough games. It's not Riot's fault that I haven't played very many games this season. In fact, they don't want to cater to players who don't play much - they want to focus their efforts on the people who play a LOT as those are the ones more likely to spend money on the game :) It's just good business sense (and if you don't play much, you probably don't care all too much about climbing - I mean I like to climb and want to be ranked higher, but I don't lose sleep over a lost game or freak out IRL because I sucked in my placements and was dropped a whole tier from where I ended last season).
And finally the fourth reason?
Those games with AFKs are winnable. As I noted I've had 5 AFKs this season. I have a winning record in those games. Easier to carry when the feeder stops feeding - if that requires him to AFK so be it (okay, just kidding, it isn't easier to carry, but it's still doable, as I've proven to myself multiple times this season).
Obviously this isn't a great reason, but winning with an AFK is a GREAT way to get in a good mindset and go on a huge win-streak. That more than anything is what makes me know I'll be able to climb, I just need to put the time in.