We point to PC gaming as a horrible state of affairs no one in the mobile world wants; but it really isn't. There were a half-dozen storefronts a few years ago. Today: Everyone is on Steam. There's EGS if you play Fortnite. I think Rockstar still has their own stuff, maybe? A couple blizzard games are still only on battle.net. Xbox has their app for Game Pass, but all their games are also on Steam. That's... it. Steam won. Its never not won.
It turns out that markets love centralization. Its an efficiency thing.
Mobile will be the same. Epic will have their store. A few others, maybe. You'll still download all the apps you care about from the App Store. Your user experience won't change (unless you want to give Fortnite a try).
What might change is: It gives a pressure point developers can leverage against Apple to negotiate better terms. Their services revenue will drop... maybe.
I say "maybe" because opening platforms and reducing prices also tends to grow the overall pie. See, there are companies like Netflix who refuse to support Apple IAP for subscriptions because the terms are unfavorable to their business. If those terms became more favorable: 10% of a million is more than 30% of zero. There are companies like Adobe and AutoDesk that refuse to build any meaningful software for the iOS and iPadOS platforms, because (in part) they would be willingly sacrificing the agency of their business to Tim Cook (for 70% of all sales; a penance). With storefront options: You might have to download the Adobe Storefront, but you'd, at least, have After Effects. No one reasonable would pick "not having After Effects" over being given the choice between using After Effects with Adobe's inevitably shit launcher/storefront and not.
Many launchers came to be, with half assed community features. They didn't want a thriving marketplace, they wanted a piece of the pie. People hated them and they died.
Recently epic store came, with a billion dollar Corp behind, and they are fighting the long battle. The free games are a nice thing (usually older offers or curated indie stuff), but the exclusive business sucks. Luckily it seems unpopular, and fewer brands esnt to adopt it - some are so strong they can do it, but others found out that they can lose their loyal costumers and fade out, eg Borderlands.