There's ABSOLUTELY no medical reason a bitch that is maintained in peak condition, is in excellent health, and will be under the care of both a veterinarian and someone who is experienced in the care of pregnant and whelping females should have ANY problem being bred back-to-back AND as late as 9 years of age, IF the breeding is done naturally, not via AI, and IF the bitch is not placed on supplemental hormones to ensure the pregnancy is carried to term. The litter size will naturally tend to be smaller, due to the age of the bitch's eggs, and there's likely to be a much higher conception failure rate, but beyond that, it really isn't as heinous as some would make it sound. Keep in mind that I'm speaking ONLY of breeding within the afore-mentioned parameters. Any animal, dog or bitch, that is too weak or in pain to be able to move about freely and comfortably ought not be pushed into service at that point.
As bad as breeding back-to-back sounds, it's actually the best option for the health of the bitch, as the uterus weakens with each estrus cycle, whether it's carrying embryos or not. From a purely medical position, the ideal schedule would be to breed the female on every heat once she's 2 years of age, always maintaining her in peak condition, and then retire her from the whelping box at the age of 7 or 8 unless she is a REALLY outstanding specimen of the breed which would warrant a longer breeding career. If a bitch is not bred in her early years because she's been proving herself in a working status during that time, it's possible that she may not even enter the whelping box until she's 4 or 5 years old, at which time back-to-back breedings and breedings into the 9th and 10th years of age are also medically defensible if she again has been maintained in top physical condition throughout her life.
While active in sled dog racing, it was not at unusual for a bitch to work in harness until she was four or five years of age before she was considered to be worthy of breeding, and there was no increase in whelping problems or health issues with those females, even when bred back-to-back; I recall one female in particular that produced one of her best litters, both perfomance-wise and structurally, at the age of ten and bred to a male who was the same age.
The success and suitability of breeding a bitch in her later years depends ENTIRELY upon how well she's being cared for, and how well her physical condition is maintained.