Ah, I misunderstood the initial poster, I apologize. He is saying the top player is no longer playing arena mode, not that they aren't playing at all. This is true of every game on the platform- I've been #1 arena at this game before (albeit for a very short time in the first season) as well as for some other games, and yeah I'm not going to play arena mode when I'm at the top, someone else will have to pass me first. It's the safest way to maintain a lead, which is the optimal way to play the competitive mode.
Also, my phrasing was a bit unclear- "No one is arguing there is no luck involved" means that there is no one, including myself, saying that this game does not have any luck. There is definitely luck involved in this game. But the elo/Arena score algorithm doesn't care about how much luck is in the game, and in fact is a great way of quantifying it- Games with less luck like Ark Nova, Azul, etc have top players with much higher elo scores (700+) than games with less luck like forest shuffle, challengers, etc, which never get into the 700 range because the luck involved makes it impossible. This doesn't mean that the "Arena Scoring is Broken", it means that players won't get to as high of an elo score before they "cap out". Ark nova had arena scores in the 2100s last season, challengers never made it past 1750. This game is somewhere in the middle. If you are the best player, you will be at the top of the ladder. It may take you a lot of games to get there, and another very good player might go on a lucky run and steal the #1 spot, but if you are good you will climb.