Well that's an interesting thought.  My Caesar presented in quite the opposite way.  My 110 lb actor-Rottweiler blew one knee at 18 months and the second at 3 years, which, even with TPLO's, put a pretty good crimp in his acting career.  As he aged, I occasionally wondered about what would happen if he god osteosarcoma, since cancer is probably the primary cause of Rottweiler deaths.  My vet told me that, because of Caesar's massive chest, and true of most large-breed dogs, he balanced the majority of his weight on the front legs and, if he ever did have cancer in a front leg, he would not be a candidate for amputation because his hind legs couldn't pick up the slack.  Sadly, at age 8-1/2 we did find osteosarcoma in a front leg and had no options for giving him any quality of life for the future.

If you did a TPLO, once Bongo started to become mobile, basically he would only have 2 legs to balance on, quite a challenge, especially at his size.  What does your vet think?  Have you considered getting him a cart?  This way he could have mobility without the pain and continue to care for his herd as every Pyr lives to do.