What separates these two aside from money?
Mainstream is about designing a game to appeal to a large audience. Instead of special interests or nieches.
Independence is over creative control - usually done by financial control. The one who pays, calls the shots, either directly, or indirectly. If your producer wants certain features in the game, that's called executive meddling, and about the surest sign, that a project is not indie.
So, there are indie mainstream games. And there are big studio nieche and special interest games. And none of those two attributes said anything at all yet about success or popularity of the games.
There is even a certain relativity to the context. On Itch, horror games are mainstream. And on mobile there are games that are considered mainstream, that you would rarely find on a desktop computer.
money you can buy just about anything
It might be more obvious how wrong that notion is, by looking at the movie sector. Is is more talked about than games. And the budgets are a lot higher and often known. You cannot buy success. You cannot spend money to make a flop movie into a non flop movie. The very definition of this is, that you get a return of investment to be considered successful. And you literally cannot buy that by spending more money. Oh, you can try and get lucky. Or fail even harder.
And one would think that those movie producers that throw hundreds of millions at a movie production would know what they are doing. Hiring the talents. Pay for marketing. Do everything by the book. And so on. And still, there are many, many flops or films that barely recovered costs.
