As the title reads, I think join-in-progress should be allowed for ranked. I’ve played my fair share of ranked games where the balance/fairness was broken because one sore-loser decided they didn’t want to finish the match. Once teams become uneven, the team that lost a player will lose the majority of the times making it a very frustrating experience for them. Similarly, being the team that has gained the advantage becomes much less fun as well because the competitive spirit is removed and in Slayer games, it becomes a chore trying to find the few players on the opposing team that remain.
Allow JIP and make it so that the player that joined is neither penalized nor awarded in terms of skill ranking. The player that joined should still be allowed to contribute towards their daily and weekly challenges. I think that would be a good way of handling the situation.
I don’t think a surrender option is a good solution. Entire teams will surrender after one teammate leaves. Imagine wanting to play some ranked matches and having to go through that scenario 2 or 3 times in a row before finally getting into a game with decent players that will finish the match.