I dont think people who have a teammate in ranked who leaves should lose elo. No one likes having possibly 1-2 people leaving, and having to be down a man, and then losing that precious LP. It's not fun, and i think that team should not have to suffer. It happens way to often for me.
Should you gain elo/lp if an opponent leaves in ranked?
Currently the system is more or less balanced in the long run over the effect of leavers, since you'll face leavers as often (actually very slightly more often since 5 potential enemy afks versus 4 potential friendly afks) as you have them yourself, so the effects cancel.
If you feel that people shouldn't gain elo/lp if opponents leave in ranked to offset that, then theres going to be a problem of people potentially having 30+ minutes 'wasted' for no elo/lp gain at times.
If you feel they should gain anyway, then the competitive value of ranked starts to slip away since over time everyones rank/elo will slowly inflate on average.
Then one person can sacrifice their elo for many, and what ends up happening is 1 person leaves so they dont lose lp
