If it's a straight comparison Arbiter vs High Khatun, in the same gear and in both cases with Seeker added, then yeah, of course Arbi is a big upgrade. I think maybe I didn't focus enough on the key point that the inequality between players is more about gear (and GH bonuses) than about the champions. If you're facing a veteran player with 320 speed on their Arbi and you have 240 speed on your HK - they'd still outspeed you if they used HK in their Arbi gear and you used Arbi in your HK gear. And the rest of their team will also consist of champions with better gear. 

For me personally, as a player in G4 who has Arbi and doesn't particularly need to worry about medals anymore (though my GH is very, very far from fully built), my biggest problem with arena right now isn't the difficulty as such, but how boring it is that choosing opponents in far too many cases comes down to a guessing game: would their Arbiter/Lyssandra be faster than mine or not? There are fortunately still some speed-lead teams that I can beat even after losing the speed race, so there is still some tactics involved in identifying those teams and adjusting my offense team accordingly, but there are also many where it's clear in advance that it's a pure speed race, whoever has the faster speed lead will win.