There's no evidence they're doing that on macOS, why would you assume they'd do it on iOS?
Mac developers have largely abandoned the Mac App Store, because it's more profitable to sell direct. Selling direct means they can sell with upgrade pricing - the thing they've been screaming for since the appstore first opened, and which Apple refuses to provide. Upgrade pricing, which customers generally support, is the basis of indy app developers' financial stability, because consumers generally don't like subscription pricing.
Piracy is going to happen regardless of what store infrastructure is in place. Piracy already happens on iOS. In a world of multiple app stores, Apple can use Fairplay as a competitive advantage to convince developers that their appstore is worth the money. Everyone wins.
That's already the case for iOS - piracy is rampant in the jailbreak scene, and malware is on Apple's app store. Piracy is a reality of any market for digital items, and you never win by building stronger locks - people who pirate, do so because they enjoy piracy, or because they didn't have the money for the app in the first place, and so were never part of the app's real addressable market.
If Apple's appstore is as good as their supporters claim, it will thrive with competition, if not, it will change to be as good as its supporters claim in order to survive. If consumers genuinely don't want to buy from non-Apple appstores, developers will find out about it, and offer apps on both. Serif sells the Affinity suite direct, or through the Mac App Store - you can even mix & match, and the sky hasn't fallen, the Mac isn't drowning in malware.
None of the things people quake in the boots about being terrible consequences for iOS if Apple lets it be more like the Mac, have happened on the Mac.