I think
Bant Nexus is one of the reasons we see for best of 3 games in Arena. There are so many ways you can beat it, but they are often not good cards in most match-ups. These are cards like
Unmoored Ego,
Syncopate,
Mortify or Status//Statue for
Wilderness Reclamation. You can see in paper tournaments
Bant Nexus often shows up, but doesn't get very many top 8 results. This is because, as obnoxious as it is game 1, their
game plan is so dependent on a few cards that are answerable that post-sideboard any deck can answer it.
Add to that the fact that most
Nexus of Fate decks on arena wouldn't even win in paper anyway. This is because, once you demonstrate a loop, you have to say how many times you perform it and then you have to do something to win the game. While
Bant Nexus without
Hydroid Krasis can still do this with ultimating
Teferi, Hero of Dominaria, exiling all the opponents lands and putting Teferi back into it's owners deck again and again until the opponent mills themselves, that's not the root that most arena players take, instead opting to just shuffle
Nexus of Fate to take infinite turns. Under tournament rules this would be considered stalling (not slow play, as that is independent of intention and would often result in a
warning) and would require the player to forfeit the match.
So, the problem isn't really with
Nexus of Fate. The problem is with two of Arena's core systems, mainly the timer and best of one matches. So, how can this be fixed? With best of one matches, the answer is if you're sick of
Nexus of Fate just play traditional and build your sideboard with the deck in mind. With the timer, however, that is something Wizard's of the Coast needs to address. The MTGO solution isn't really amenable, as if we just switched the timer for that, people wouldn't even need to take infinite extra turns to waste the opponent's time, they could just stall by taking longer on each decision within a turn. I think the real solution could be to keep the current timer, but also to have some kind of definition of a player doing something, then if the player doesn't fit that definition for a few turns they are warned, and if it continues they forfeit the match. Now, this definition of doing something might be a point of contention, but by working with the players and the community I believe that there will be a solution we can come to.
Sorry for the long post. TL;DR: It's not
Nexus of Fate's fault, just play best of three, and push for Wizard's to introduce a secondary timer.