I'm not sure if the arena matchmaking really depends on account power. Is there any real investigation about that? We don't know how the matchmaking in arena works, and the speculation of a few players is not a proof.
It's kind of illogical that you get stronger opponents in the arena by such a mechanic. The arena matchmaking definetly is not coded to punish the player EGDNIT. If your account power raises by buying shards, it will be the same for everyone else. How comes that the TEAM POWER of your opponents increase by your ACCOUNT POWER? Other players (your opponents in arena!) open shards too, they level up champions they use at CB and in dungeons, but not in arena, too. How comes that only your opponents in arena get harder? The player xy you had in your enemy-list in arena yesterday opened the same number of shards as you did. If the way of matchmaking works like you saied, he should be in your enemy-list again. And where do all the "harder" opponents come from? If that would be true, shouldn't there be some threads like "arena is so easy now. It was a hard job a year ago, but now all the bloody noobs are in my arena tier"? If you get harder opponents now, from their point of view they get weaker opponents.
In addition to that, the opponent's team power is not really an indication how the fight will end. In a speednuker team, a champ like High Khatun does nothing than giving her speedbuff in the first round. With lvl 40 she can do exactly the same as with lvl 60. But an opponent with a lvl 40 Khatun will probably have a much lower team power than an opponent's team with the same champs at lvl 60.
I've investigated it enough to be confident about it. Take that as you will. If you can find any other reason for why across multiple samples over multiple days the relative account power remains more or less the exact same no matter what the absolute power is, I'd happily hear it. I would also like to hear an explanation for why my arena opponents shift so dramatically in power level despite my tier remaining the same or even dropping.
And no, my situation is not unique, and I'm certainly not the only one who has noticed it as you can clearly see if you look around in threads about the arena. Many have noticed that they are getting strong opponents, and many have noticed that they have started getting worse for no apparent reason. You ask why I should be the only one affected, and that's a fair question, but why are you so sure others are not? But let's run with that assumption, and I can think of a few reasons for why I'd be a special case.
1) I don't throw away my champions. Even those I have no direct use for, I don't get rid of because I may find a use for them later, or maybe I've overlooked the use I might have for them now (quite likely). I currently have 85 champions in my vault that I have no direct use for, in addition to food of various ranks. Most of them are rares, a handful are epics. Others may get rid of them where I don't.
2) Others may have experienced the same situation and just given up because they find that their progression is blocked. I've been sticking with it because I'm stubborn and was lucky enough to get Serris, who I can say is pretty much single-handedly carrying me. Others may not have been as lucky. Or perhaps they got even more lucky, and are now one of the teams I get to face.
3) Others may pay for better shards and skip straight to epic/legendaries by bypassing the rares entirely, thus never having to stock up on rares or having no need to save them because there are legendaries that supersede them.
That aside, would you expect to see people complaining if they found the arena too easy? I'm going to go with a hard no on that one. Instead, they would move up the ranks and be happy. However, maybe they are not very active in the arena, and thus drop for that reason when similar teams to theirs target them and they lose, but they don't fight enough to climb up again. In fact, I've heard time and again from people up in the gold tiers that the arena is indeed very easy, and that they have no trouble staying up there even without any defense. A YouTube video I watched even went as far as to say that's how you're supposed to do it, unless you're going hardcore for platinum. This is another issue with the arena, but one separate from matchmaking. The offense/defense aspects of it makes advancement asymmetrical, in that if you're too strong you don't get points for defending, and if you don't attack you won't either, thus your rank is to some extent less a function of your potential and more one of your activity.
And yes, I'm well aware that power is not everything and that power is in itself an extremely arbitrary number (as I touched on previously). However, it is not without any meaning, and when you face a team 2-3x your power full of legendaries, well, odds are not in your favor. That being said, I routinely fight, and quite often win against teams with 50-75% more power than mine with 1-2 legendaries in it, if I pick my fights carefully. Not because I want to, but because that's the best I get, and that's regardless of whether I'm in B4 or S2. Like you say, a speed team doesn't need high power, I'm not sure if that is because speed is either valued very low compared to other stats, or just outright ignored in the calculation, but it's quite misleading. That doesn't make it a better metric for matchmaking though, it makes it much much worse.
