Aside from everything already well written about these changes, I would like to add my thoughts on some styles of games that have not been touched on much.
Games with a large amount of luck, getting to the top takes a good streak to get there and your elo is a reflection of how lucky you got. Once you drop from the leader board for inactivity your elo remains as a reminder of how lucky you got at one point. Sometimes I will play a game many times over hoping to one day get lucky enough to make it near the top. Some games I enjoy and keep playing after getting to the top and some games I take a break from because I've played more than I wanted to just trying to get up so high. I enjoy being able to see my elo where it was, and like that it drops off the leaderboard so others can now try. Farkle currently has a top spot of ~340. If someone looks at my profile they can see, wow he sure got a nice lucky streak going to have that elo. Making it decay seems to solve nothing, and will just erase what we can see, which would be disappointing.
If i return after being absent in a luck based game with 100 elo more than the current first place, and I lose, my elo is earned by the community as it should be. If my elo decays, it does nothing for anyone.
For co-op games, elo decay may be insurmountable. In those games elo is more an experience indicator, and removing some of that provides no purpose. Someone who is a master and has no elo visible dropping 70 points would take 70 games to get back to that point in games where a victory awards 1 elo. If that happened to me, I probably would just give up playing that game.
In co-op games where adding difficult variants gives higher elo for success, hiding the elo shown/gained just makes it no longer worth trying for harder scenarios. Playing regicide with harder rules was fun because you felt good about earning more when you were successful. If everything just shows as a "win" there feels, personally, less motivation to play harder rules, and therefore less motivation to play that game/be on this website. Also for regicide as an example, someone with 700 elo and 4000 elo, are obviously different in amount of games won and familiarity with the game. But now with hidden elo, it's impossible to tell the difference.
Power leveling should not be enabled in arena or tournaments where you cannot select an opponent. But I also think they should not be enabled in normal games either. As others have said, small player bases, time of day you play, etc, all will effect playing against the same opponent. Why bother having a rematch button if it penalized you for a rematch.
To someone's suggestion for needing to play every 3 days, sorry but this does not work for long games or people who play turn based. I have 30 tables of some games at once, but they may all take weeks to complete, so therefore I would miss the cut off.
If you win against someone (assuming luck based game for now as elo swings are normal) and you are both elo 200, then the next day you face them again and are 150 and they are 300, you will get penalized because the last time you beat them. Mind boggling.
In luck based games, i might have a 50% win rate, and just from the luck of the draw I have most opponents with 50%, some opponents with 100% win rate, and some with 0%, it's just the way it goes. Why are we penalized for being lucky against the same person?
Again as others have said, elo leaderboards only show active players, arena resets quarterly, winning more than once already lowers your elo because the elo difference is greater after each win, seeing elo is important for deciding play styles and as a nice way to see your and others dedication to a game.
Good job on fixing team based games no longer winning/losing based on your teammates tho, that's a positive.