The fear of Pyo was the reason for considering spaying. I still couldn't make up my mind until the mammary tumor. A next female will be spayed much sooner than at 6 years old.I had to deal with a horrible case of pyometra in our Aussie. She had mostly silent heats, no problems. We were diligent in keeping her away from intact males while she was in heat, so no opps litters.
Then she got pyometra and the surgery and recovery was hard on her. She was seven years at the time and was very sick. The guilt I felt and the worry I had was enough to change my mind about spays. Yes, let them develop. But don't put it off like I did. Once they hit two years? Unless you have competitive requirements to keep them intact? Spay.