There are three main categories of entry into a device via zero-days: WhatsApp/Signal, SMS/MMS, and Firefox/Chrome/Safari. If these can be isolated, entering a device could become harder.
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Edit:
Something like this, but for phones
https://learn.microsoft.com/en-us/windows/security/applicati...
As for iOS, to my knowledge it doesn't allow for any such app segregation.
In general, we need stronger per-app isolation such that a zero-day affecting one app doesn't grant any access to anything else.
(Microsoft and security are distinct concepts, btw.)