It becomes everyones user experience when every big company only offers their software on their own app store so they don't have to pay anyone else a cut.
Yes, but is that inevitable? Facebook tried to take advantage of Android's open nature in the past and failed outright, and that wasn't even an app store but an app launcher (Facebook Home). The Amazon Appstore is mostly used for their Fire Tablets anyway. Ditto Samsung Galaxy Store is specifically for their own devices. Epic's attempts at building their own mobile app store hasn't gotten anywhere. So what other phantom third party app store are we so afraid of? Steam?
Here's a small but more poignant reason: F-Droid exists on Android, not as a huge app store, but a nice FOSS-only community with its own standards of app quality and screening for malware. Surely as hackers we can at least appreciate an analogous community being made feasible on iOS, and not in a slightly more shady unofficial capacity like the jailbroken-only Cydia.
And surely, as hackers, we can appreciate having the freedom to not be unilaterally subject to the whims of a single corporation's dictates.