I don't see how replays would help you defensively. The way the AI plays your toons in Arena would be drastically different than the way you play the same characters. The more complicated the kit the harder the AI is at dealing with it. Your human instincts might say hey I need to take out thrawn or ability block boba asap where as AI would be like fuck it ill just hit the tank. not debuff the taunt and kill an attacker.
So replay's offer no valuable data other than hey this guys strategy worked so I can use it elsewhere. Which isn't super helpful either because youtube is full of videos that offer suggestions on how to beat a certain team.
I also don't like the idea of an free cost of transiting offense toons to a defense one just in the hopes of keeping your position. If you want a defense arena team after you gained position re-attack and potentially lose with your defensive team and waste a turn. Giving you a free Offense and Defense for the same attack would be heavily abused by Whales and Dolphins.
I just straight up wished the AI was better