My squad sort of proves how bad defense is (and also why I'm still going for other characters).
Arena squad that you'd all laugh at Level 70 - all 6* Sid (L), RP, GS, Phasma, and a 5* QGJ. All max geared.
My arena power is 23K. I face teams that are easily 4K or more higher in power and some matches I still have all 5 standing at the end. Some may be near death (QGJ especially) but I won. All the teams in my ranking (200-300ish) have 7* level 70 teams. There may be a few with a 6* in there but it's mostly 7* teams and all are level 70.
My highest level toons are 6*. I have Lumi, Poggle and others up there as well but all stop at 6* I always stop and go after somebody else. Why? Honestly I dont feel like grinding out 100 shards each. I've got near 7M in credits and lots of droids. I can finish GW against the 7* level 70 teams with my deep roster (and I just got Barriss which makes things infinitely easier).
Honestly I am happy where I am at in Arena. I am on an old shard and I am sure the top teams have full meta, max everything. I still get shredded when I try my luck against a maxed Rey/Yoda team or other top tier. Starring up my top squads I really dont know if it would really gain me much in the game. I am sure the top 50 or even 100 are a pretty crazy in how fast rankings swap around.
I thought I would have to go to 7* and as the game gets older and older with caps going up and other things changing maybe I will but I think it really shows how bad defense is as well as how you dont "truly" need a 7* team to go far in the game. You'll never hit rank 1 and honestly probably not top 100 but you'll be able to progress along just fine.