The problem is none of those are workable. They create more problems than they solve:
[{quoted}](name=proud and bronze,realm=NA,application-id=cIfEodbz,discussion-id=h0tlR4Pb,comment-id=0003000000000000000000010000,timestamp=2014-12-30T17:30:18.212+0000)
give the remaining players a 10% dmg boost
So you've built an early lead, and your team has managed to take some good objectives. You've put them on the back foot, and are slowly building your lead into a win. Suddenly, 3 of them come out of base and randomly melt your team.
I guarantee that if a system like that was implemented, the boards would erupt with complaints about how unfair it was.
put in some kind of replacement player system
The limitations that would have to exist for such a thing would ensure that replacements either a) were rarely of an appropriate mmr, or b) took so long as to be irrelevant.
Alternatively, you have to recode tons of stuff to allow players access to champs they shouldn't have access to.
let a bot take over the afk players champ
Honestly, I suck at this game, I really do. And I can personally 1v5 Intermediate bots, without any real trouble. A system such as this would actually be worse than just letting people leave willy-nilly.
make one of the players on the other team leave for winning IP
I don't think creating artificial IP inflation would be helpful. In fact, I think it would create entirely new problems. Moreover, you'd still never see it used in Ranked unless it also gave winning LP - which would be even worse.
That's not to say that they shouldn't examine other options. But I still haven't seen one - from you or anyone - that was really workable. I've thought of a few ideas myself, but they either create new problems, exacerbate old ones, or simply don't actually address the situation.
It's not a function of how League is built. It's a function of how MOBAs work, and short of completely changing the game's style, I don't see a real solution.