Arena is one of the most fun game modes and has a bigger player base than even Conquest. I grant that there is less planning skill involved. The only planning required is considerations of safe wave clear and cooldown timings. Raw mechanical skill is a far bigger variable in Arena than in Conquest due to lower proportion of planning being a variable, but I don't see how this dissuades the game mode from being competitively viable.
The few Arena tournaments that took place in the past (like Alienware Arena Cup) were very enjoyable to watch, so it's certainly viable from the perspective of spectator enjoyability.