SwiftUI and Catalyst are not mutually exclusive.

I started a new iOS app recently, and there was essentially no cost to building for catalyst.

I am using SwiftUI and I considered targetting MacOS directly, but the extra work wasn’t worth it at this stage.

I think catalyst is the new Cocoa - it will grow to consume all of MacOS.

This if anything makes Catalyst more important.

I'd be surprised if the iOS apps did not work on Intel based macOS. Project Catalyst already works on Intel macs and my guess is many iOS apps have been migrated already.

I'm sure there will be emulators. But I'm also fairly confident that they'll be relatively slow. Emulating across architectures is rarely performant, and if Apple had solved the problem they would be talking a lot about it right now. In the past they've gotten away with this because the architecture they're moving to was so much faster than the previous architecture that even with a 50% or 75% performance penalty the apps would run faster than they did on the old hardware. With this new hardware it is likely only going to be marginally faster than the old Intel chips since the focus is more on power efficiency, so emulated apps are probably going to feel sluggish.

The difference is that the entire (64bit?) iOS App Store back catalogue would likely be available to run as unmodified binaries without developers having to lift a finger.

I think we’ll know more after the Platform State of the Union