Luke Wroblewski's 2009 mobile-first design philosophy looked like salvation when smartphones started eating the web. Google pushed it hard. By 2016, mobile traffic had overtaken desktop.
Nobody saw the desktop problem coming. Optimizing for small screens first doesn't scale up gracefully. It creates "content dispersion" on larger displays: bloated text, screen-covering images, excessive white space, endless scrolling. Accordions that collapse information efficiently on phones become interaction nightmares on monitors, fragmenting content without the space constraints that justified them.
Mobile is 55% of web traffic now. That other 45% navigates interfaces designed for a different context entirely. Every responsive system must solve for two fundamentally different interaction models, not just different screen sizes. The infrastructure cost compounds daily.