Plot inconsistent with technology

Every once in a while I read a SF where technology described makes the
plot absurd. Star Trek Next Gen is the worst offender -- almost every
problem they ever encountered could be easily solved with a magic
device introduced in some OTHER episode, -- but is far from the only
one. SF books are generally better about this sort of thing than movies
or TV, but I had seen some real bloopers. Probably the worst example
was a longish short story (a novella, I suppose), whose name
unfortunately escapes me. The story dealt with a boy growing up aboard
a generation ship as it neared its destination. The ship community had
some rather harsh laws; in particular, everyone who reached age 60 was
euthanised. Most of the story revolved about shipboard politics, which
were very nasty -- people faking their age, others uncovering them,
some false accusations of age concealment, etc. Eventually the
protagonist discovers that, unknown to everyone else, the ship carries
a centuries-old Caretaker -- a man who does not age. His job is to be
"the hidden hand" -- to ensure ship community does not stray too far
from its destiny. I forgot whether Caretaker had finally reached the
end of his life, or there was some other reason, but he gives the
protagonist an injection and... turns him into new Caretaker. Keep in
mind, that happened only a few months before planetfall.

Huh?? If the civilization involved has the technology to stop aging
permanently (or at least for a few hundred years) WITH A SINGLE
INJECTION, then why did they ever bother with a generation ship in the
first place?? With all nasty politics, loss of institutional knowledge
and loss of purpose? A crew of immortals would reach the goal star
without any of these problems. Even if immortal Caretakers are
infertile (which the story did NOT state, or even imply), a cargo of
frozen sperm and ova would take care of that. What was the author
thinking?

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