I think a book with a Summoner class, A Beastmster class, an artificer reprint, a new caster, and a new martial would sell better than most setting books.
Not releasing new classes was a choice to not be beholden to a rapid schedule of the past. But in that choice, they have crawled into getting to many traditional and nontraditional classes, subclasses, monsters, and options.
The "Well the only things worth it for WOTC to publish were published" is a sill argument. By that logic WOTC shouldn't have made TCOE because I designed a horizon walker class on my own. I'm sure many of the official mechanics and products were made by 3PPs before WOTC got to them .