I hate when people quit or AFK and you end up losing your ranked game because of it you shouldn't lose LP or if you did lose half not the full amount we as players cant control the actions of another person and we shouldn't be punished for it as well riot fix the issue and a ban doesn't help if its last game of your promos and someone leave and you lose so you have to start over again
Because Riot thought Leaverbuster, Teambuilder Draft are more important.
Everyone dislikes having to fight the uphill battle that is a 4v5 with equally skilled opponents but there are a couple of reasons for the way the current system is used.
I think the most appropriate example I have seen is the analogy of ranked being a gamble. You bet your LP and time into a league game and are dealt your team and opponents. Sometimes your team just sucks or has an afk, likewise the enemy team can also have these hindrances. Once you have been dealt your 'hand' you can't just demand your bet back because you got shitty one. You take a risk with your LP every time you queue up.
Aside from this the actual system is ill equipped to dealing with AFK as LP is redistributed from the losing team to the winning team to resort ladder rankings (Example 1). The rankings remain roughly the same area as LP is redistributed roughly evenly each game. If as you suggested, teams with afks receive halved LP losses the system has points unaccounted for and effectively generates some new points for the winning team (Example 2). This can lead to a problem of inflated ranks as missing LP from an AFK mercy rule allows people to access promotion series' sooner than normal as losses lose less LP than wins gain. This can lead to people have extremely low MMR for their rank as they have been inflated by AFK games. Yes the current system might not be completely fair and accurate depiction of your player skill but everyone has an equal chance of an afk and adding in an artificial mechanism is only going to further distort any attempts to provide more clarity.
The second point, and one more focused on by most people is it's abuseabilty. Any afk mercy system is very likely to lead to a small subsection of competitively minded individuals to engage in unsociable behavior in an effort to get someone to afk and obtain a lower LP loss. This could be outright verbally abusing a teammate who is not playing well and demanding they afk for the good of the team or if the person is smart and has them muted then being generally uncooperative with that teammate in an effort to get them to ragequit. You yourself pointed our that bans don't help in game and the increase in general hostility in solo queue is not what is needed.
As a matter of personal opinion I don't think an afk mercy rule should ever apply to a promotion series, if Riot is going to have these artificial gates on skill they should atleast stick to them.
Example 1: Team A has 1350 MMR and Team B has 1300 MMR. Team A is expected to win. The following outcomes can occur. i) Team A wins and receives a pat on the back LP gain (10-15LP per player). Team B on the other hand only losses a similar amount of LP (10-15 per player) ii) Team B wins and receives a bonus amount of LP as an underdog prize (+20-25 per player) and Team A losses similar amounts as they were expected to win (-20-25 per player)
Example 2: Team A has 1290 MMR and Team B has 1310 MMR. Team A is expected to lose. Team B has an AFK. The following outcomes can occur. i) Team A wins and receives a small amount of extra LP for beating the odds (18-22LP per person). Team B loses would lose 15-20LP per person however the system recognises the afk and halves the losses to 5-10LP. What happens to the 25-50 LP that is missing? ii) Team B wins and receives normal LP gains (15-20LP per person). Team A losses about the same LP per person.
