I wonder if you could still have free to play, but just had a limit to how many different things to buy. Say, you could buy everything for $60. That way, you don’t have to worry about someone spending thousands because they wouldn’t get any more than if they spent $60.
Compare this to a traditional game like Cities: Skylines. You pay once for the base game, then there are various DLC expansions that work as you suggest. You don't need them to complete the game, they just add new features and can only be purchased once.
Edit: 'Pay to win' is a better description for most games in this category (and Clicker Heroes 1 looks to follow that), although there are some free to play games like Dota 2, which have DLC that just provides cosmetic changes and isn't needed to win.