It might not have been enough. But WOTC made the "no new classes" decision before 5e was printed, then 5e's demographics changed rapidly.I doubt 2 additional classes would be enough. Some people want "crunchier" Fighters, some people want mythic heroes, some want a 4E fighter, some want fighter that can compete with bards and rogues for skills.
WOTC has done and continues to put significant effort into playtests,market research and surveys (not all of which are open to the broad public). They have a better idea of what works than either one of us.
WOTC did a lot of playtests,market research and surveys. But they locked themselves into "printing PHBs like madmen" before the situation changed, then got to the point their were to sunk into the idea to change... anything unto the 50th anniversary.
Crawford even admitted that they sat on ideas for years and were just waiting to display them in the 2024 version.