So, this “size doesn’t matter” is indeed a lie 
I understood how it works, technically. No problem about that.
After, is this trend really a demand from consumers, or pushed by providers? (and consumers are believing it’s what they want afterwards). Like mobile apps, I find there are more advantages for providers than for consumers.
I guess with phone numbers, it’s easier to identify people and to prevent sockpuppets and such. Someone banned will have a harder time creating alternate profiles, etc. So, I can see the advantages provider-side. Consumer-side, not that much. Even less than for mobile apps (where there are some). Especially as you can install an app on your phone which will popup a notice when you receive an email, the same way than when you receive a SMS. I don’t see what advantage SMS brings to consumers over email, except psychologically maybe, but even there, I have a pretty hard time to really understand the logic.
I don’t know how it works in Asia, but in most of Europe, you can’t have a phone number without giving an ID. In the US, I am under the impression it’s becoming increasingly hard to get a basic “pay as you go” sim card where you can receive calls, with no monthly costs (where you would only pay “as you go” when you make calls/send SMS). I see that most providers seem to now push for monthly “plans”, even if still prepaid. All this compared with something pretty free and which can be created with no or little privacy concerns (I’m talking about email, of course). SMS length is also quite limited when you want to send a message.
I don’t see either the logic in not wanting an email address or not having one, for just when you need one. Is it easier for your wife to have one to give when asked or to have to answer each time that you don’t have one (knowing that it will block you sometimes). I can understand not wanting a phone or a credit card. This often has costs and can track you. I can perfectly understand not wanting a computer and managing everything with your smartphone. This actually makes a lot of sense to me (you may try to propose to her to connect an external keyboard, mouse and screen for when she’s inside the house, but it may not interest her).
Anyway, i don’t know. That businesses want people to use their phone numbers, I totally understand. I would be pretty much against it, personally (from a user POV). That people got used to this way of doing, I can also understand. That people have a problem creating an email address, here it becomes difficult.
As for allowing people to do this if it isn’t your own goal, I’m not convinced. Sure, more options are probably good. But in the end, it doesn’t seem easier to me nor to really bring any advantage. I can take the point that “it’s the trend” and why “fight” it rather than “go along” with it. But still (and here, it needs some development to enable its use).
PS: “Email is quickly becoming a relic of the past for a large segment of the market (the younger generation).” Why not, but compared to SMS!? Please. SMS is for sure as much a “relic of the past”, if not far more.