Unfortunately, if "tweaking matchmaking" is all they are doing, then it doesn't really solve much of anything, it only changes which players get screwed over. I'm guessing I'll be one of those getting screwed over, or at least I probably should be, and I'm just hoping that I get something in exchange for it. Probably not though. Thing is, it doesn't solve the issue of punishing people for collecting and equipping other champions than just their arena team, it doesn't address the fact that the arena is and will continue to get progressively harder for new players, and it certainly won't do anything about the meta to make participating in the arena anything more than a boring grind, at least until you actually have enough champions that you can do something other than hoping you're faster than the other guy.
If they were actually smart and/or interested in making the arena palatable, they'd introduce the bots that they're denying ever existed. Not as free wins, but as teams with reasonable difficulty for the tier they are in. It's a very simple solution that'd at least alleviate the problem somewhat. I don't think advancing in the tiers is necessarily the problem (although getting out of bronze should probably be easier and the missions are badly out of sync and should be looked at), but players should be able to find matches that are reasonable.
Generally speaking, a 50/50 success rate is about where you want a player to be at; enough wins to encourage them to keep trying, and enough losses to prevent complacency. Ideally, a player should end up in a position where that is the case, and remain there until they have what they need to advance. Of course, players should also feel that they aren't stuck there forever, which means you need to counteract the overall power inflation of players, and do something about the matchmaking on player power so that people don't feel like they are going backwards by developing their account.
All-in-all, Plarium has created perhaps the most frustrating pseudo-PvP system I have ever seen. If that was not their intention, they are almost impressively incompetent. If it was, well, then I have some far less kind adjectives to describe them.