Permabanning someone for going AFK? You seem very rational.
I don't condone the idea of leaving the game of your own volition. If someone evidently leaves due to a ragequit, trolling or griefing and such, then I think that justifies an appropriate report. Generally speaking though, this is the only reason I will report someone for being AFK. If they left at an odd time or clearly aren't having a brilliant time with their internet, I'll give them the benefit of the doubt and assume that something happened that was unexpected or out of their control. When I report someone, I'm aiming it at the person who owns the account and their attitude, not the activity of the account on its own.
But I fail to see why a permaban should be distributed to someone who goes AFK for only a few times. You may consider going AFK to be 'the most toxic thing you can do', but this is hardly true. People go AFK for reasons that aren't fueled by malice. Intentionally feeding is done to purposefully to be a detriment to your team, and harassing your team through flaming is actually what 'toxicity' is.
Life happens, and Riot already acknowledges this, but still dishes out punishments accordingly: ones within reason. A permaban is completely unreasonable however, since the lives of each players will sometimes interrupt them in the middle of a game:
-Parents telling their children that they need to eat or do chores.
-Emergencies where you have to stop playing and take action.
-Internet or power going down at your place of residence.
(and that's only to name a few)
Yeah, we should ideally have a schedule to adhere to, leaving room for people to play the game without interruptions. Whilst I'm away at university, I don't have my parents breathing down my neck, so I can play a game when I have the time without any interruptions, whilst at home, I have to consider what my family is doing and what time it is before queuing up.
But life doesn't always work like that. This is why issuing a permaban for inactivity is not a thing, whilst it is intentionally feeding or flaming: because the latter are easier to assess. The chat and the champion behaviour are a lot more reliable at seeing who is toxic on a frequent basis. For leaving the game, the intention isn't always apparent, therefore there needs to be some leniency towards people who go AFK, otherwise the carefree handing out of permabans would be completely unjustified in many cases.
Quite frankly though, I think that this mentality you have towards AFK players is both extreme and petty. I lose some LP because my game became a 4v5: such a shame in the grand scheme of things. I'm not saying that it isn't frustrating: because it is. But it's the epitome of a first world problem.
There are people in the world who are desperate for food and water, people who suffer from famine and illnesses and people who are victims of war, yet you have access to a computer and free time to play a nice game and are crying about something as insignificant as people not always being present during a game of LoL. For real?
If you can't accept that not everyone on your team will always be there until the very end, every single time you queue up, then stick to normals, or don't play at all. This discussion is not going to reach Riot's ears and make them change their policy on punishments towards AFK players.
Life will often overlap free time - including games of LoL. If someone has to leave, the most you can do is submit a report and hope that Riot takes action from it, but you don't get to dictate whether people are or are not allowed to play a game that they enjoy.