What if Bigfoot didn't need a significant breeding population?

Here is an extremely unlikely, but entertaining theory on Bigfoot, breeding, lifespan, and population.

---Hydra (Genus)---

For those with an interest in cryptozoology and Bigfoot (a.k.a. Sasquatch or Yeti), there are various theories out there as to what Bigfoot might be.  One theory is they may simply be an unidentified upright walking ape.  Another is they may be ape/human hybrids, or even just another tribe of humanity that has kept themselves hidden from the rest of us.
 Some think they may be interdimensional beings or even visiting aliens.  And, of course, there are many that are skeptical of their existence at all, assuming the stories are just made up, hallucinations, or misidentifications.

We don't know much about interdimensional realities, if they even exist at all, and the idea that they may be interdimensional beings seems far-fetched...at least within our current level of knowledge.  There's little evidence that they are alien visitors either, besides circumstantial evidence that some sightings of Bigfoot have corresponded with sightings of UFOs.  And if they are simply living flesh and blood beings, then it is assumed they would need a significant breeding population to be able to sustain themselves as a species...and if a significant breeding population is needed, then skeptics question why they remain so elusive.

But what if they didn't need a significant breeding population?  Would they even need to breed at all if say, they were....immortal?

Immortal?  You think that sounds pretty far-fetched?  Yeah...actually, I guess it is pretty far-fetched.  But maybe we could consider that they live incredibly long lives and have trait known as negligible senescence (also see this cool video about aging).  Senescence is the process of becoming old.  But there are some organisms that exhibit negligible senescence, meaning they don't appear to age after their initial development into adulthood (or more specifically, they don't lose reproductive ability or have a measurable functional decline with age).  Hydras are even thought to be biologically immortal, meaning it's thought they only die due to injury or disease, and not through deterioration with age.  Lobsters also appear to exhibit negligible senescence and may even be capable of living indefinitely, barring injury, disease, or capture.  Thetortoise is also known for long lifespans.  Some plants are known for longevity too, like the Great Basin Bristlecone Pine.  The Methuselah tree is estimated to be over 4700 years old.

So what if Bigfoot existed and exhibited negligible senescence?  What if they could live indefinitely...barring injury, disease, or capture?  If they were a community of biological immortals, perhaps reproduction would be limited or even unnecessary, and maybe they would choose to avoid us because it would put them at risk of being injured or captured?  So what if instead of needing a significant breeding population to sustain their existence, theres actually just a few hundred of them around the world total, and they are the same ones people have been seeing for many years?

Ok, so maybe the idea that Bigfoot could be biologically immortal still sounds far-fetched.  But what about the possibility that they just live exceptionally long lives?  What if the average lifespan was 1000 years and they rarely reproduced?  It could still explain why they are so rarely seen.

But then again, there's all those other theories mentioned above too.  And, of course, it's also possible that they really just don't exist at all.