iPhone and macOS are basically the same product technically. The reason iPhone is a single user product is UX decisions and business/product philosophy, not technical reasons.

While plausible deniability may be hard to develop, it’s not some particularly arcane thing. The primary reasons against it are the political balancing act Apple has to balance (remember San Bernardino and the trouble the US government tried to create for Apple?). Secondary reasons are cost to develop vs addressable market, but they did introduce Lockdown mode so it’s not unprecedented to improve the security for those particularly sensitive to such issues.

> iPhone and macOS are basically the same product technically

This seems hard to justify. They share a lot of code yes, but many many things are different (meaningfully so, from the perspective of both app developers and users)