I personally view the issue as less "it wasn't in D&D before" as "it makes the game more complicated and the worlds more crowded". In the same way adding four or five brand new humanoid races is problematic for campaign settings and adds more options bogging down character creation. Suddenly you're explaining how shapechanger wilden fit into Eberron.
Some of the existing classes could probably go bye-bye as well (barbarian, ranger, paladin, and sorcerer) but survive due to tradition. Newer options don't have that same history, the "well, it was in four other versions of the game..." impetus to keep it around.
Yeah, it's easy to say "well, don't permit that option in your game." But then WotC is creating content they expect a large number of people to ignore. Making ignorable options is fine for a DM book with rules options (especially ones that only take a page or two) and have far more limited testing. Classes should be playtested out the wazoo at multiple levels, making them very work intensive.
Classes take a lot of work to make, so it shouldn't be done unless there's an expected high usage. In the 5-pages a new class occupies, we could have a like 10 new subclasses.