I asked a fairly well respected vet the question and received this answer _
"If you spay a female dog (or cat) b4 she EVER comes in heat, she will never get mammary cancer, which is the most common cancer of female dogs, just like in people. It is fairly protective to spay after one heat but after that the protection factor fizzles out. Obviously a dog spayed at any age won’t get a pyo.
As long as they are responsible (and smart) owners who will not let her get bred (you’d be surprised at what “smart” people think will work), they will probably choose to take a small risk and wait".