There is water, food and smartphone. Those are the only three things that have (or soon will have) 1:1 penetration for the world’s population. It makes stuff like tablets and watches look like piddly little businesses, even though those are huge businesses by any other standard. Apple alone sells millions of units which generate billions in revenue every quarter.
There will likely never be another smartphone. Its reach and scale are impossible to replicate. But it doesn’t mean there isn’t 20-50 multi-billion dollar “niches” out there that companies like Apple, Samsung, and Xiaomi can profit from, and the combination of those “smartphone ancillary” products combined might be as much revenue as the smartphone by itself.