I am still playing, I also made it to Diamond III, and also suffer to win battles, but I will keep on playing. I am currently only renting out my double cards, but the thought crossed my mind to rent out all my cards. This could actually give a better income than playing!! (I see now after the end of season the rental values dropped quite a bit, causing some cards not to be rented out and I had to adjust prices.)
This is what I was doing too until now. It offset the rentals I took myself.
Start of the season, people need different cards and at different levels.