Meaning league will one day become irrelevant, an object of the past.
While sure, this is possible, there are many games that maintain playerbases (and even unofficial support) for dozens of years. Look at roller coaster tycoon, the sim city series, and many other games that have gone well over 10 years and maintain a relatively stable (but smaller than peak) playerbase. The future possibility the game ceases to exist is not a factor in deciding how to maintain the game today.
Our sentiments are whats giving our accounts meaning. The hard grind, the joy, the anger, etc. Once league fades into the past so will our attachment to the game. There's also the possibility of someone moving on in their life and simply quit league. Thus we don't know what will happen down the road in 1 or 2 years. That's why a ban of such length is as significant as permanently banning an account.
So you're saying that because it may not be a thing in 2 years, a 2 year ban is akin to a permanent ban because everyone's going to sit here and think "well, it may not be around so it's as good as gone"? That's a... fairly abstract and uncommon train of thought. While it may be your train of thought, most people will think "welp, got banned again, time to hop on another account and set a reminder on my phone/google/apple account/whatever for 2 years to go back to my normal".
Furthermore, what would this say to people who get a 2 year ban and are able to come back to their account? It'd say that Riot will never permanently remove someone - which is contrary to their goal. Riot does not want people coming back, ever, at all, when they're permanently banned.
There's also a second thing i want to point out. We change as time passes so 1-2 years later our personality might shift into a more mellow way.
Riot has run currently two separate experiments separated by a couple years where they unbanned accounts that had been banned in some cases for multiple years. The first of these had over a 95% failure rate (i.e. over 95% of players kept misbehaving), and the second abruptly stopped enrolling people and there has been no changes from it, suggesting it was a similar result.
Sure, some may change. But the bottom line is that most people who are toxic in a video game to the point of a permanent ban will never get out of their mindset that "it's behind a screen" and "there's no harm done" and "it's okay because i'm frustrated" and the like.