When the "mobile first" design pattern really took off is when the Internet was handed to the masses IMO. That's when it became accessible to everyone's mom and grandparents through their phone and that really changed things from a user base perspective. At the same time, all the mobile apps started getting published and the internet, to most people, became answering the question "which app do i tap on?". Honestly, it's pretty much that way for me. The Internet, outside of work, consists of this website, my youtube app, my wikipedia app, spotify app, and maybe google news app.