The standard model is something like $100k up front + 30% of sales. If someone's net worth is in the dozens of millions or more, this really isn't moving the needle, so the major incentive has to be the altruism.
However, will people continue to choose to be altruistic on this platform, when it is turning the creators into the ultra wealthy? One might be less inclined to altruism if you're actually giving a +1 to the world but a +50 to a random company who hosts a website. Many of the people that have done Masterclasses could make an arbitrary individual wealthy and famous (e.g. Spielberg could just say "<random no-name filmmaker> is my protege and all films I work on will be co-directed by them"). The question is how many people are going to make that choice for Masterclass when they could just as well host it on their own website, or put it on their own channel on youtube, etc.