A modern iPhone sure but what about the iPhone 2/3G? That’s basically a low power ARM chip and we have Nintendo Switch emulators with very good performance. Anything pre-security chip should not be that difficult to emulate.
Many older apps may use undocumented functionality or non obvious quirks of the system that an emulator may miss, which means you'd need to have a QA team testing individual apps for compatibility.
Part of Apple's brand is usability and a lack of rough edges. The downside of that is that building a tool like this up to their standards would be prohibitively costly.