In "The Neutral Zone", the Enterprise happens upon a number of 20th century individuals who have been frozen as sent drifting in space to be hopefully found later and healed. They are easily ... unfrozen and healed.

But Data states:

> 
> Actually the process of cryonics
>  was never more than a fad, and
>  did not continue much beyond the
>  mid-twenty-first century.
> 
> 
> 

How is it supposed to be plausible that there existed the technology to freeze people in the 20th and 21th centuries and the technology to revive them in the 24th century, but no one ever uses it? There ought to have been many cases where it would have been useful to freeze people to be thawed later. And are we to believe there were no people in the 22nd century who wanted to "live forever" and would have taken the same route after reliable power and space travel were commonplace?