The secret is to see why people bring 3PP books to the table. Is it because the class fits the theme of the homebrew setting? Do the monsters? Or does the player want the class because it is so overpowered...you'd be surprised how few times that reasoning is given and how often role players will sacrifice their combat capabilities for a well-rounded character.
Often the 3PP offers the tools for GMs to create homebrew settings and the players the means to play in them. My biggest product, Ultramodern5, simply offers a toolset to create non-fantasy games.
Want space opera? We can do that.
Want mecha in your 5E game? We can do that.
Want steampunk? We got that too.
There are many games people want to play which required 3PP products to do so.