It's interesting that people who never search solo don't support party matching, while players that search solo (at least sometimes) do. It's almost as if searching in a party of selectively-chosen players in a team-based playlist conveys an advantage, and furthermore that players who have such an advantage don't want to lose it...You have it backwards. We're not gaining an advantage by teaming up in a team-based game. The way I see it, we're the ones playing the game correctly. Players choosing to play a team-based game without a team are putting themselves at a disadvantage. That's on them. I encourage every solo player to start searching with teammates and stop putting themselves at such a disadvantage.
You might be right if only players who already had parties were allowed to register for multiplayer, and searching solo were then an option. In that case, they would be putting themselves at a disadvantage. However, that's not the case. Players don't inherently start out with a party, and furthermore, there's nothing explicitly in the game that requires a party. Furthermore, don't confuse team-based with party-based. Remember, Bungie both H2 and H3 had party-matching in all ranked playlists because Bungie understood that it was only fair to match parties against other parties of the same size. That's not to say that MCC BTB should be ranked, but it is insightful about the nature of parties in team-based games. You're absolutely
gaining an advantage by partying up with a chosen team of other players rather than searching and getting whatever random teammates the matchmaking system assigns to you.
Also, it's not really realistic to expect solo players to find party teammates, especially on a thinly-populated game in a dying franchise. The Halo player base numbers have gone through the floor, and expecting new players to develop those connections isn't going to motivate them to stay. Solo players aren't putting themselves at a disadvantage; players joining a party are gaining an advantage.