I don't think a cap is the right solution. Better would be regulations, controls, etc to enable competition for marketplaces (at least for those of a certain size)

Like on iOS, if there was actual competition for app stores, Apple would face pressure from a competitive market to reduce their commission percentage. Developers will not want to put their apps/games on Apple's store if they continue to charge outrages fees, and they have alternative, realistic distribution options.

However, I don't know what regulation would look like there. Even if Apple is forced to allow third-party stores, it could still end up like the browser wars, where one company manages to secure a monopoly anyways. If we can't even fix the browser situation, I don't have high hopes for the appstore market.