You play through to the bitter end.
I highly doubt that you're going to play a game when 20% of your team or more doesn't even appear to the game at all or suddenly walks off the lawn because they stop caring mid game. I at least never heard something like that ever happened.
Maybe that's because the players are not assigned randomly to the teams like in solo queue.
And that's exactly the breaking point: In preformed teams, the team carries the risk of it's members as a whole, because they can decide whether they take a player or not. In solo queue, that's not the case at all and thus, you cannot blame it on the team for having such a player.
Besides, you are arguing against the wrong end. The team with the advantage would be the one interrupting the game and trying to make it less lopsided if it was interested in good sportsmanship, not the one with the disadvantage. They would bench some of their own players or lend some to the other team. Needless to say, if it's about millions of dollars, good sportsmanship does not really matter very much anymore.
Sportsmanship is all about fairness, not about "giving your best and shake hands afterward". That's why you don't let cripples compete with "intact" people, that's just not sportsmanlike. According to you, it would be perfectly fine to put everyone, men, women, cripples, kids, lightweight, heavyweight etc into the same league, just shake hands afterwards. You gave your best, tough luck you got no chance to begin with.
Sorry, that's a very twisted idea of sportsmanship you got there.