The Cavs won 1 of 4 finals against GSW due to Lebron's greatness. If they had better defender's they might have won more. Besides the poor defense, you ignore that Love is 30 with a big, long contract and quite injury prone. There is also no guarantee that Butler won't be a 1 year rental. If I'm the Sixers all things considered, I much prefer Chandler to Love.
The Cavs didn't have any issue winning a title with Love's defense and they had Irving and Smith starting with him (and it isn't like Thompson is a great defender and James had already started his defensive decline). I think the defense argument is vastly overblown. If the Sixers win a title it will be because of their offense and the match-up problems a monster like Embiid is in the post and a gigantic point guard like Simmons causes (along with Butler's defense on the opposing great wings like James, Leonard, etc.). But in order for that to really work, they need the floor spacing that Love and Redick provide. Offense quite simply is far more important to winning basketball then defense in the modern game. You have to be able to hit 3 point shots or you aren't going to play effective winning basketball in the postseason.
So you propose a trade to ensure that their defensive doesn't get better. Coach Brown is all about defense, pace and space. Love only fits only one of those criteria. The Sixers are trying to compete for championships which requires solid defense. They'd easily get bounced in the 2nd round again with Love and Redick in the starting lineup.
I wouldn't do that trade if I'm the Sixers. Love and Redick in the starting lineup would kill their defense. Maybe, maybe not, but it would make their offense significantly better and frankly they aren't defending all that well right now anyway.
The Sixers should save their ~20M in cap space for the offseason. I expect there to be some nice players available at the trade deadline and they still have their room exception.
If they did that trade and Butler leaves, they will still have max cap room, or at least very close to it as they would only have Love, Embiid, Simmons, Smith, Shamet, Bolden, and Korver's buy out on the books. They could also keep McConnell's cap hold on the books, have their 1st round pick (if they get Sacto's that would complicate it), and could then just have the empty roster cap charges of like 800k to get to the minimum roster. That leaves them max or just about max cap room depending on what the cap actually ends up being. And that essentially means they acquire Love for Fultz, which I think is a net gain for a team trying to compete for a title (and also is a net gain the other way for a team that is rebuilding).