Give it enough time and all defenses will fall out of diamond. Mine’s at 4200ish and I usually hover in the high 2500 or low 2600 for cups. Not playing much for a day dropped me into the low 2400 cup range.
Having 5* do not ensure you to stay in diamond but give more opportunity to move forward. As I m raiding with 4* and a tp around 3500, I need more luck to get throught 4200 tp team than if i had a bunch of 5*.
The point is getting these juicy food and iron from diamond chest as many time as possible and not stay in diamond.