Fistulation surgery isn't all that dangerous or expensive. The major expense is in keeping the rumen port clean and the cow healthy and free from infection afterward. It's a significant amount of work and consumable supplies, that has to be done by a human worker, every day, for as long as the cow has the port.

Granted, a gas port should be smaller than one large enough to stick your hand through, and pull stomach contents out of, but even so, an entire herd of ported cows would not be able to pay for the additional ranch hand work necessary to keep them alive. You need a port-cleaning and health-monitoring robot, at minimum.