Forcing Apple to allow third party payments without Apple's cut would improve market opportunities for many businesses. Facebook could have its marketplace conduct peer to peer transactions. Amazon could allow the purchase of digital goods (books, movies, etc.) and put it on more equal footing with Apple itself. While big businesses are best positioned to take advantage today, the effects directly trickle down to small startup businesses.
While I personally don't care for it, cryptocurrency use would have more potential. Apple blocked apps for NFT features in the past because they couldn't get their 30%.
Having third party marketplaces might make it so that there is some actual curation at the App Store.