ELO was designed for single-player games like chess, and favors champions who can hypercarry and essentially ignore one's own team performance
That's quite a claim, and I don't think it's accurate.
I strongly disagree with the idea that the win/loss system favors hypercarry champions. This assumes that those champions win more than other champions or win more in the hands of skilled players. Neither assumption is true. Players who use team-based strategies can and do win just as much as other champions. A player who is skilled with Janna can have just as high of a winrate as a player skilled at Zed or Vayne.
The beauty of the the win/loss system is that it only cares about what is effective. If the game supports team-based strategies as a way to win, then players who use those strategies effectively will be just as rewarded as other players.
The flaw of the current system is that it measures skill slowly, but it ultimately measures it accurately and fairly. I will take that over an inaccurate system.
wondering if we can't do any better than win/loss
Theoretically yes, but not with the tools we currently have. Developing them would likely take much, much more work than it is worth.