1) Developers already raise the prices for apps/subscriptions in iOS (vs, say, the web) to offset Apple's cut.
2) Developers will be able to offer refunds more easily (something that continues to be ridiculously hard with the App Store).
3) The App Store only allows free trials for subscriptions. They don't offer free trials before a one time payment to buy the app. A 3rd party store could allow that.
4) Certain kinds of apps that Apple doesn't allow on the App Store would presumably be allowed on other stores.
5) Apple will finally be forced to compete now that the App Store has competition. It might even mean that all the points I made above become moot.
I'm not saying there won't be any potential downsides, but that's the whole point of competition, it's a tradeoff you can make. As a user you can choose to only use the Apple App Store if you want. Since most users will only use the official store, it seems unlikely that many mainstream apps would consider dropping it entirely.