However, US monopoly law is based on consumer harm, so...
I think on a mobile platform, with plenty of competition to not buy an iOS-based phone, a walled garden absolutely benefits consumers more than it hurts. There is so much PII on phones now. Given the total lack of InfoSec knowledge, especially at the mobile-user level, a walled garden is crucial: see every Google Store vuln that hasn't hit Apple.
the harm isn't consumer oriented, since it's somewhat diffuse, it's about concentration of market power in the industry.
we could live in a world where you target one distribution platform, and phone vendors compete to police malware, but instead we have walled gardens that police content & economically lock you to their environments and don't even do a good job about malware
That's true, but that's not how anti-trust law works in the US unfortunately. It's consumer-harm based.