I have had 3 Mastiffs with crutiate tears.
The first two tore as seniors, because of bad winters where they lost muscle tone. The third is my puppy, who tore because she is very large and played too hard with her light, agile friends, on immature soft tissue (mistake on my part).
The first was 11yo. We opted to do CM, and spent a small fortune on her at a rehab clinic - massage with laser, passive ROM, water treadmill, cavaletti, and rocker board. (She was also treated for spinal stenosis and a bicept injury from pulling herself along). She was very active - hiking in the forest, and doing laps around the farm everyday, until a month before she was PTS at 13yrs 4 months.
Second was her daughter at 10yo. CM again at saem rehab clinic. She remained active until her death just short of 12 yrs.
Current puppy is now just over a year old. She was doing well with CM, but due to puppy antics, became very unstable again (likely a close to full tear now). In addition to going to rehab, we invested in a laser, and a pool. DH does massage, laser and Passive ROM every night. We just started her in her pool - set to walking height for her, and in the three days since we have been using it, have seen a great improvement in the stability of her leg. We will not do surgery on her until her growth plates close, and only if she is not stable by then.
IMO,(and the opinion of the physical therapist) whether you do CM or surgery, the best way to avoid arthritis is to do PT. Passive range of motion, massage, laser if you can get one, and water therapy (walking in water is better than swimming), and whatever exercises the dog can tolerate without going backwards.
I have never heard of a Mastiff having a leg amputated for a crutiate rupture, that seems like over kill. TPLO is the surgery of choice for a Mastiff (according to my ortho vet and PT). Actually I remember theh PT telling me that once the TPLO fails, it is very, very bad, perhaps amputation was what she meant. She also said that recovery after surgery was very important and if you vet just tosses the dog in a crate after surgery, there was a good chance of the dog injuring himself, struggling, as he woke up.