It still could. Many people buy their kids cheap tablets now because buying them an ipad is too expensive, and later in life those kids will be familiar enough with the Android ecosystem that they may never switch to Apple devices.
If those same parents could instead just hand off their ipad to their children and have it only load their apps this could lead to them being more familiar with ios and buying apple devices later in life.
Not a guarantee obviously, but it could theoretically have an impact in future sales and market share.
I'm not sure what you mean by that, since what I'm implying is that there is no real economic incentive for Apple to support multiple users.
And on the other hand, there are also benefits (for Apple) of encouraging one-user-per-phone. For example, it makes it more likely it becomes an extension of your identity. Having multiple users per phone undermines that type of personal attachment.
This isn't something that started with Apple - mobile numbers have always been tied to individuals - but it's very convenient for their "lifestyle" approach to selling their units.
Mobile phones will never be shared by people who don't already share them. This is a convenience functionality, not something that would change how you use the device - from the exact reason you said: mobile phones are extensions of identities. This feature is something you would use when your own phone is out of reach (e.g. on a shelf in the living room) and your wife's phone is with you in the kitchen.
True, it will not exactly boost sales, but it will not decrease them. It will make some people more likely to recommend Apple. Everyone will still have their own phone.
You can overcome the risk of decreasing the likelihood of creating personal attachment by letting the foreign user log in to a de-personalized (no custom wallpaper and so on) space and use a limited subset of functionality, e.g. a browser, contact list, the Apple messenger app and a phone app (that would call from your own number/phone over VoIP); this functionality would be available only when both phones are connected to the same wifi.
I don't see it as being that cut and dry. These new features would have to be focus tested, designed, tested, rolled out, and tested some more. There are maintenance costs for it, as well as additional configuration to present to the user. Done poorly, this sharing option might be simply ignored by the user making the above a waste of time and resources that could be spent elsewhere.
If those same parents could instead just hand off their ipad to their children and have it only load their apps this could lead to them being more familiar with ios and buying apple devices later in life.
Not a guarantee obviously, but it could theoretically have an impact in future sales and market share.