Is Cancer a feature or a bug?

I think we've got cancer all wrong.  It's not a bug, it's a feature.  Death is a basic requirement of evolution.  The current generation must make room for the next.  Without generational turnover, the population grows exponentially until resources are depleted and the species dies out.  This requirement has existed since the dawn of time, and different mechanisms exist to enforce the requirement of death.

One mechanism to ensure mortality involve telomeres (the aglets that hold DNA strands together) which shorten with each cell division.  My supposition is that the various forms of cancer are inherent in our DNA.  At different times during our evolutionary history, different groups of people (or earlier mammalian ancestors) developed different cancers which gave the group or family (but not the individual) an evolutionary advantage.  How could this be true?  Imagine two equivalent tribes.  One tribe develops, say, lung cancer that occurs in 25% of the (elder) population.  Over time, that tribe's age will skew younger, and fewer of the tribe's resources will be devoted to supporting the aging members of the tribe.  A younger tribe may have a competitive advantage over the tribe that did not develop cancer.

The common understanding is that cancer is caused by chance mutations that cause cells to turn off their natural inclination to die, and instead, divide willy nilly.  However, if this is so, then how is it that tumors manage to grow their own blood supplies, or trick the immune system into ignoring them?  Cancer is such a complex phenomenon that it seems more likely to be a feature designed by evolution than one that occurs purely from chance mutations.  Perhaps it lies lurking, requiring some chance mutation (or damage to DNA via telomere shortening) to activate it.  It exists because it serves (or once served) an evolutionary purpose: to cull the herd.

In a pre-literate culture, the elderly have a great deal of knowledge.  If all die young, information and wisdom critical to the survival of the group may be lost.  So a disease that is probabilistic, that only removes a percentage of the elder population, is preferable to one that wipes out everyone over a certain age.

Note that there are certain animals, such as elephants, bowhead whales, and the naked mole rat that do not get cancer (or get it far less frequently than humans).  In the case of elephants and whales, perhaps there is value to the knowledge held by older members of the group, and that knowledge trumps the advantage of death.