Phones dominate not because of screen size but because the operating systems they run are fundamentally more user friendly. iOS/Android were the first time experienced OS engineers were able to go back to the drawing board and rethink the whole stack from scratch. They learned a lot of lessons from what worked and what didn't on generalized desktop computers and were able to build something people prefer using.

That's a rare opportunity. I hope someone gets a chance to do it for professional workstation computing one day. The Windows codebase has IMHO reached end-of-life. The Linux community isn't going to stray far outside the bounds of the 1970s era UNIX design. Apple does OK but they can't/won't change the macOS UI in any meaningful way and not enough people write native Mac apps to justify trying to do anything interesting there. ChromeOS is basically just a large screen smartphone OS experience with some Linux virtualization on the side. We're caught in a local minima.

It's mostly screen size... Life happens, and of the two, the phone is the one with you. Wherever I go, i carry my phone, but not my laptop. I take pictures with it, so they're stored there. I take screenshots, and share stuff with friends, gets saved on my phone. I browse on the phone on the bus/subway wherever. Browser history gets saved there, opened tabs, etc.

Bringing photos, saved stuff to computer all takes an extra step. For most of the time I need a digital device, the phone is good enough, and most importantly - it's with me. Not just when i get home.

When high productivity is needed, then yes, laptop is king. But if somehow we could have the same productivity on a device, 10x smaller that I can carry with me, why wouldn't I start doing everything on that device? That's why smartphone is king.