You inferred that it wasn't possible when you posted exactly what I quoted you saying:
"But who do you think is going to fill up empty spots in party matches?"
You then said: "It's been like that since UC3 MP", which is true of the UC franchise, but TLOU is the more recent Naughty Dog game (and pretty much the exact same type of game in terms of what we're discussing with multiplayer deathmatch, party systems, etc.).
I've played TLOU MP for years on both PS3 and PS4, and while I'm sure there were issues, party segregation was not widespread enough to consider omitting the mode/option. I think there've been party segregation issues in numerous current gen games I've played (in Destiny I've wound up on the opposing team during deathmatch, in Rocket League I somehow got put on the opposite team when playing Chaos mode a few times)... but I'd much rather have parties occasionally get split up and/or face slightly longer matchmaking times than getting thrown into matches where a party is just dominating solo players to the point where it's not even close.
I understand that there's nothing to necessarily protect from blowout matches (even with rank/level-based matchmaking) thanks to the occasional smurfing or drunk players or the way she goes, but a party of 4 or 5 vs a team of solos is going to win the vast majority of the time.