Stores have nothing to do with "user bases." You just want people to buy stuff. As a publisher, you have all the leverage when it comes to pricing arbitrage. You can sell game X on your platform for $45 and sell the same license to Discord for $50. The consumer doesn't care where they buy it from, as long as it's cheaper. Obviously, they're incentivized to buy it on your platform.
I'm not saying that platform size doesn't matter, but funneling resources for a win there would be a Pyrrhic victory. Steam won that battle a long time ago.