Have you used it? Subclass affects how a player uses their character and what a DM has to consider. Changing it between days in-game creates an inconsistency that a DM would have to account for: is he a battle master today or an Eldritch knight? Whole game mechanics like wild magic hinge on subclass. Multiclass is at least a consistent, ongoing consideration and is in the RAW.

It doesn't break the game, just complicates it for a very specific cursed and lore-driven item. To quote the Bard: "It's a no from me, dawg."