Climbing rank on offense is easy, not dropping 20-points overnight is the issue. I sure as heck don't want to fall from 1st to 20+ overnight and have to work my way back up. All teams are beatable at this point but there is a difference between a team that will drop 10-points overnight and a team that will drop 20 or more overnight. A squad with Thrawn is that latter. It may not matter to some people but it does matter to others, defense is the most important part of Arena. Having a good defensive squad also helps against payout snipers that hit people from first outside of their payout times.
Arena may only be part of the game for many players it is more important than raid. For those that prefer raids that's fine, but you shouldn't discount those that feel Arena is more important. The AI doesn't matter for raiding because you're not auto-battling through if you're trying for a strong score. But in Arena we want a more effective AI for defense.