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.
This was my thought as well. Initially "Starship-Class" referred to capitol ships. Over time popular usage was to refer to any Starfleet vessel, and later and FTL craft of a certain size. As the popular usage changes, the official nomenclature changed as well.
Its not the best rationalization, but it is a way to tie the way the term was used in much of TOS to the way it was used later in the franchise.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy