I still think Join-In-Progress should be kept… but revamp quite a bit. Lets think of this logically the main idea behind JIP is to keep teams even, matches ongoing and the overall game fair… so it has a positive benefit.
How did we used to deal with the issue of quitters, lost connections or other non-active players or even trolls… punishment via experience, and thus demotions… perhaps we should have one and also the other.
Leavers, as in quitters and lost connections, should be punished somewhat if they have quit more than two matches in a row, during matchmaking. They would be punished via a grandous xp take or perhaps a straight demoted rank… and if the quitting continues like 10 matches over a period of 2 hours or so… they are temporarily banned from matchmaking.
In terms of the stayers, current players on the now unfair team, should first have the option of forfieting the match, which turns to an overall vote… and must be unnaminous… to which the match ends the opposing team wins, gains experience as usual, and the other team just return to matchmaking lobbys without punishment.
There is also the other option if they elect to continue to play with low numbers… the experience they gain multiplys by the disadvantages they attain… so one player less, 1.5 times normal xp, two players 2x, three 2.5x, four 3x etc. The join in progress option still remains available but only for lobby searchers, to which before they join they get to check the stats of the match and elect to join or pass… this way they have control of the outcome of their K/D and xp… also the benefit of a join-in-progress would allow some xp gain even at a loss… take for example the original players on your team lost, but you JIP… you gain 0.75 of the normal xp… thus giving you some reward for the risk.
Also if you have idle players, you should be able to team elect to boot them if they do not perform any commands during the phase of the match… perhaps even going so far as to temporary ban from matchmaking lobbies.
I think my suggest way of punishment, reward and risk of experience and the JIP… is a much need successor and advancement of the matchmaking