the term has been used far too widely since then for far too many types of ship.
But only long after it was used in the dedication plaque of NCC-1701. Back in those days, it got no competition from humbler ship types. So, terminology mutation.
No; in-universe, the term was used generically in the 22nd century according to Enterprise
and in the mid-23rd according to the 2009 film; and in real life, the term was in use in science fiction as early as 1926
, and was pretty common in SF titles such as "Sargasso of Lost Starships" (Poul Anderson, 1952), Starship
(Brian Aldiss, 1959), Starship Troopers
(Robert Heinlein, 1959), The Wizard of Starship Poseidon
(Kenneth Bulmer, 1963), and so on. In reality, it was a generic term for ships capable of interstellar flight long before there was a Star Trek
, and it stands to reason that those works of fiction would've existed in the Trek universe's 20th century as well. So the use of "Starship Class" as some sort of unique designator was always awkward. It's well-forgotten. (Although, for what it's worth, I can't find any movie or TV titles using the word prior to the 1970s, at least on IMDb. So Trek may have popularized a term that was previously only common in prose SF.)
It's not healthy to be too fixated on the details of a work of fiction, especially its earliest installments. Not every idea is a good one, and the creators of an ongoing series are the first to be willing to renounce and correct past mistakes. Clinging to old ideas that the creators themselves have renounced is not something that most creators would want their fans to do.