Sequel, Serial, Series

(i) The first form of fiction has to have been the single story, spoken not written.

(ii) Next, there were sequels by the same author or by a successor. Homer's Iliad was followed by his Odyssey, then by Virgil's Aeneid.

(iii) Greek dramatists created trilogies, whether thematic or linear.

(iv) Nineteenth century periodicals serialised novels before their book publication. Here again was a single story but now read in installments.

(v) For periodical publication, Arthur Conan Doyle (I think) created the series: a serial in which each episode is a complete story so that a one-time reader gets to read an entire narrative while regular readers appreciate continuity. Series synthesises single story, sequel and serial.

Even those who do not follow prose fiction series are all too familiar with the series format from TV. In fact, everyone knows of Star Trek even if they have never seen an episode. Thus, the basic story, Kirk and Spock on the Enterprise, is known even when particular stories are either not known or forgotten.

It is impossible to read some works by James Blish, in particular Cities In Flight, without remembering Star Trek. In Star Trek/Cities In Flight:

a starship/city flies to various planetary systems where, however, it must not upset social systems;
we see a handful of individuals in controlling positions but not the entire crew/population;
we are shown the next generation (Star Trek: changed political alliances; two starships; a space station. Cities: a new interstellar empire; the astronomer's daughter and the city manager's grandson; a planet flying between metagalaxies; the end of the universe.)

Wherever they differ, Cities is superior - more imaginative with better characterisation and a sounder basis in knowledge of the relevant sciences. Where Star Trek does excel as a series, of course, is in sheer number of episodes:

three seasons of the original series;
a prequel series;
three sequel series;
the animated series;
the feature film series;
in other media, novels, comics, even fanfic (amateur fiction written by fans).

Quantity is the antithesis of quality. In fiction:

talent creates quality;
individual prolificity or team script writing generates quantity.

Thus, I suggest that:

James Blish created several works of considerable quality;
Poul Anderson, both talented and prolific, created many works of comparable quality;
Star Trek comprises many more works of mediocre or at best variable quality.

The ideal quality-quantity synthesis would be an sf series as long as Star Trek but as good as Cities In Flight or the History of Technic Civilization but when will that exist in reality?