Keyboards, screens, mice, network terminals, speakers, microphones, radio antennas, gyroscopes, GPS receivers, cameras, etc, etc. Over time computers integrate everything they touch into themselves.
Specialisation at the component level (radio chips, GPUs, GPS chips, etc) is fine, but modern smartphones often have some or all these features integrated onto the same silicone as the CPU anyway.
Who wants to carry a handheld games console, a handheld GPS, a phone, a pager, a camera, an alarm clock and a palm computer? I used to routinely carry a London A-Z map book. Not long ago these were all naturally considered to be separate functions requiring a separate device. Smartphones are perplexing to you because you think they are phones. They are not. They are computers.
But there will always be a market for specialized and interchangeable parts, as they're will be people who need that level of control to do what they need to do. ie a professional photographer will always use a dedicated camera because quality, ability to change lenses etc. are crucial for them to be able to accomplish what they need to do.
Cost in terms of: time spent, energy used, will power needed to get it done, the 'need' cost, and lastly money cost.