It's the convenience, for me anyway - why carry around multiple devices, when one can do the job.
For travel photography, I went from carrying around a Sony full frame, to a Fuji XT3, to hoping by iPhone 19-20 that I can sell all my bodies and lenses and just rely on the iPhone.
The Sony felt like a chore - from carrying around a big camera and lens, through to the editing and photo management. The Fuji was a breath of fresh air - a bit more compact, and the film sims allowed me to cut the editing process out. But there was still lugging around a camera, and then the photo transfer etc.
With mobile phones' improvements in photography, coupled with the endless opportunities for apps, I can't wait to rely on it as my sole camera.
What do you expect the iPhone 19/20 to have that the iPhone 17 doesn't?
The tradeoffs will fundamentally be the same. You're not getting a > 100mm^2 sensor (with the glass lens required to image on it) on the iPhone anytime soon (or anytime ever really, unless the form factor of the iPhone dramatically changes).
Hopefully 48MP telephoto lens, with similar aperture to the main lens.