When do you think smartphones will become obsolete?

Never. A better question is how will smartphones evolve.

Phones are humanity's magic crystals which enables everyone to send instant communication channel to anyone else and tap in the greatest common knowledge base ever. I don't think there is a reasonable future without everyone having his own personal device for this function. Maybe it won't called a "smartphone" but it will provide these functions.

It'll get unbundled into AirPods + Watch + Glasses and maybe further down the line Brain Implants will come into the mix.

It won't be going away for a long time though. It's here to stay a while.

A smart phone is just a programmable computer welded to a cell phone, that's here to stay. So I'm going to answer what I think your real question is: "When will smartphones change form factor?"

The current form factor is a best-we-have compromise between ease of use, and utility. I think the next big thing will be the decoupling of utility and ease of use. AR/3D gesture tech in a discrete google glass-like thing maybe.

Books never became obsolete. It s just the physicality of it. Having something in your hands allows you to focus. Or else, if the screen was your glasses, what would you do with your hands?

When AR (already happening) and human brain/AI symbiosis happens!

Possibly before the neural implants if IT becomes so ubiquitous that there's no need to carry around a phone.

When any surface you look at or touch will interact with you. There will be no need to put anything in pockets.

When UI is so slow and so full of useless notifications that you can't use it to answer quick questions.

When a better(greater functionality, easier to use, etc.) convergence device becomes available.

When glasses with display become usable.