I think that differing device resolutions and aspect ratios in some ways forced designers to think about things that were always worth thinking about but were e

I think that differing device resolutions and aspect ratios in some ways forced designers to think about things that were always worth thinking about but were easier to ignore. I use a tiled window manager, I care about whether your desktop site is responsive even at minimal widths because I tile windows. I also have a touchscreen monitor hooked up to my desktop and I like to be able to use it. And sometimes I also full-screen windows on a 32-inch screen and use a mouse-and-keyboard. If a desktop site accommodates me in all of those scenarios, it'll probably be fine on a phone as well. But it was so easy in the past to just ignore that and treat PCs like they were uniform devices used in a uniform way, and phones meant that you suddenly had to care about what a website looked like in a single-column view, you couldn't just tell your users to maximize the browser window. Unfortunately, rather than taking away the lesson that design should adjust to nonstandard situations, layouts, and input-modes that can not be fully predicted or tested for in advance -- instead developers took away the lesson "okay, now there are two standard devices we have to support: mobile and desktop." The distinction isn't real, there is no hard line between a desktop and a mobile site. There are mobile tablets that are big enough that they should be served a desktop layout, there are desktops with touchscreen displays, there are monitors that are 3/4 ratios. And there never was a standard and computers were always like that, but it's an understated truth that every developer and every designer would secretly love to develop exclusively for consoles with integrated screens and one input method, and developers often kind of behind-the-scenes somewhat resent the fact that general computing is an open ecosystem with diverse devices. So designers often just treat computers like they have two completely discrete interfaces, or at worst decide that because they're not targeting one of them that they now have permission to target exactly one resolution and size again. Sometimes that takes the form of designing "mobile only" like the top-level comment talks about and calling desktops a dead platform. Sometimes it means designing desktop only and getting mad that somebody flipped their monitor vertically instead of horizontally and now wants the ability to move a side-drawer to the bottom of the screen.