Bigfoot's needs and abilities are always stretched into whatever shape is needed to fit the narrative. <--- This almost always makes the narrative wrong.
-Say Bigfoot is sick, suddenly there are more fellow Bigfeet out there bringing him food.
-Say we can't find a body, Bigfoot bury their dead
-Don't have sightings in decades of looking, Bigfoot has the ability to teleport.
When Questions about Bigfoot arise, we need to look at the most likely scenario. Sure, shows like Finding Bigfoot seems to know what Bigfoot's favorite baseball team is, or his favorite color. Not bad considering they have never found Bigfoot let alone studied the actual issue in Queston.
Yes, we can imagine various scenarios for Bigfoot. But if bigfoot is old or injured is just more likely to die or get eaten by another predator. Nothing extreme needs to explain it. If the food supply dries up for Bigfoot in one area, like most animals (or people) it would move on to an area where it needs are available. Billy the Kidd (of whomever) robbed banks because, "That is where the money is"
Simplicity rules. Simplicity is the marketplace of nature.
Brody: Now this guy, he... he keeps swimmin' around in a place where the feeding is good until the food supply is gone, right?
Hooper: It's called "territoriality". It's just a theory that I happen to... agree with.
It is easy to imagine elaborate scenarios to explain all things Bigfoot. When Bigfoot is not able to meet its needs, it dies. It will live so long as it can. We don't have to imagine much beyond that