Saying phones are stealing market share from PCs is like saying that shoes are stealing market share from cars because people spend more time wearing shoes than driving cars. They're different devices with different purposes. Phones are OK for media consumption but terrible for anything creative (other than maybe photography.)
I think the thing is that 90% of computer/internet users are media consumers and very few are media creators, apart from as you say - photos.
Writing the occasional email is clunky on a phone but the experience of using a phone still beats worrying about your backup, the location of your photo files, the OS license or Antivirus on the PC.
"People in general" simply don't write large documents, programs, create music etc. They email a friend, browse instagram and take photos. The smartphone form factor happens to be the perfect match for this usage pattern. So much so that if there was a fraction of the use cases they can't do on their phones - people much rather just skip doing those activities entirely and stay on the smartphone.
