I would be remiss to not point out that Skip.dev exists to fill exactly this role, enabling the creation of complete iOS/Android apps with Xcode in pure Swift. If the Swift SDK for Android is analogous to KMP, then Skip would be the CMP equivalent (except that unlike CMP, Skip has the advantage of a native UI on both platforms and no garbage collection imposed on the iOS side; see also: Swift on Android talk at iOSoho Meetup (w/ Compose Multiplatform)).
Every app is different. Indie developers and small teams will benefit greatly from building their apps with just a single language, IDE, and technology stack. Sprawling and complex apps with large teams will often be developed with modules in multiple languages and technologies. This split is most often seen between the UI/Logic layers, where the UI is created with SwiftUI/UIKit on iOS and Jetpack Compose/Views on Android and the logic is shared in a single implementation layer (Swift, Java, Kotlin, Rust, C++, whatever), and the two layers are glued together somehow, with a fluency that often benefits one platform over another (e.g., C++ bridging is smooth for Swift but painful for Kotlin; Java bridging is smooth for Kotlin but painful for Swift).
To circle back to the main focus of this thread, some people assert that a Logic layer shouldn't need internationalization, and thus FoundationEssentials is sufficient and the size inflation from FoundationInternationalization is a non-issue. In my experience, unless we are talking about an extremely limited "Logic" layer (e.g., pure algorithms or scientific applications), any logic layer without i18n support for the handling of numbers, dates, calendars, currency, pluralization, and collation would be so trivial as to undercut the benefit of partitioning the app into separate languages in the first place.
I think we can all agree that a smaller app size is better, regardless of how it be architected. Reducing the i18n overhead for non-Darwin Swift apps is a particularly thorny technical issue, but I think it is eminently solvable and a worthwhile endeavor.