If you ever get a hold of the novelization of Star Trek The Motion Picture,
published thirty years ago this year (oy), the only ST novel written by
the Great Bird of the Galaxy Himself, Gene Roddenberry, check out the
early chapters.

Roddenberry describes humanity in the 23rd Century as moving towards
living in family groups or collectives where everyone shares the duties of
either being the parent or child. He referred to the starship crews with
their monogamous relationships as "throwbacks" to the old days since
they were away from Earth so often and isolated on their vessels.

So while this may not directly explain what happened to the monetary
system in the ST universe (then again, Uhura bought a tribble for 10
credits and Spock said he is worth over 122,200 credits to Starfleet),
it would appear that Roddenberry tried to promote his view that we
would all be living in a commune-type society in that future. Though
judging by the vast majority of the other ST films and series, this type
of society didn't seem to get very far.

It would be nice if everyone shared, didn't need money, and never fought,
but I think by the time that happens, we won't be human any more.