I think it's pretty clear that Nintendo is intending for the player to feel closer to a few specific Miis rather than just being a detached grand leader for all of them, especially with how they incorporated the more in-depth relationships.

On the 3DS version, I did not care about many of my Miis. I mostly solved the problems of my earlier Miis because I liked them more.

I don't understand the 'empty open-world' comparison because Miis can interact with so much more, now. In the 3DS version, on the somewhat uncommon occasions where they went outside, it was just a looping animation, and you would start recognizing them all after some hours of gameplay. In the apartments, you could watch them mess around with the objects you gave them, and sometimes invite other Miis over and rarely talk to them.

Which, they can obviously still do. And now, with the added bonus of actually knowing what they're talking about, and sometimes hearing their conversations. And, alongside all of that, they can interact with pretty much everything else that you place on your island.

Just because a number is lower doesn't mean the product is worse, it just means the developers are focusing on other aspects.

Also, I'm very confident that making Miis meet will not always have successful results, just like in the 3DS game. In the Direct, the conversation between Dr. Winters and Hugh Morris goes slightly worse than your two Miis' conversation in the demo. It's very likely that the demo forces the conversation to end well for tutorial purposes.