In your scenario, Apple morphs iPadOS and iPad hardware into a Mac. In my scenario, Apple switches the Mac to ARM and modifies macOS.It seems less likely to me that we'll see ARM-based Macs as much as we start seeing iPadOS-based products rolled out in form factors that resemble laptops and desktops as the primary offering for mainstream consumers. A smaller selection of Macs would remain to keep specialists and old-timers happy, probably at a luxury price point. This seems indicated by iPadOS's increasing computer-like capabilities, such as "desktop Safari," file management, and external device support. iPadOS already gives Apple what they want – a closed, highly secure, revenue-generating platform, nearly identical in both usage and third-party support to their phone OS, with a robust touch-screen interface to compete with Windows-based products. Making iPadOS Apple's primary consumer operating system also spares them the pain of having to actually transition macOS to a new architecture; why modernize a legacy product? It's easier to gradually sideline it, as iPadOS establishes itself as Apple's OS of the future.
Either way, Apple ends up with ARM running on hardware with the Mac's ports and internal hardware.
From my experience, switching the Mac and macOS to ARM would be the path of least resistance. My previous posts detail why I believe modifying macOS for ARM is not that great a task.