"Desktop" encompasses everything from ultrawide 8k monitors to 768p laptop screens where the user may have scaled down the browser window - and that's on brand new hardware, I know people who are still using 15+ years old laptops!

This alone means you either handle any window size and size ratio or your UI will break for some users.

> This alone means you either handle any window size and size ratio or your UI will break for some users.

You should spec your UI in device-independent units and let the GUI handle things like different pixel densities (or aspect ratios) and font sizes.

There are many different screen resolutions. Being able to adjust the application based on available space makes the application usable to more people.

And it introduces new failure modes. I often experience responsive web applications hiding UI elements in my Firefox windows. Those windows usually use either the left or right have of a 28" 4k display. Why the heck do the hide the sidebar to save space? It is fucking annoying.

In theory that sounds nice but in practice i haven't seen a single desktop application that can actually handle resolutions lower than whatever resolution the developer/designer used. I used a 1366x768 monitor for a long while ~3 years ago and everything looked both gigantic and had padding everywhere.

As for the web, responsive design was so great that in almost every site i had to zoom out to make it think i had a monitor with a bigger resolution than i really did.

These days and since i do not maximize the browser (because my monitor is huge - like all modern monitors that do not have awful image quality tend to be), i often have to resize the window because sites tend to think i'm using a mobile phone and instead of scaling down / hiding less important stuff (that would at least be appropriate for a narrower viewport) they make thing ultraginormous (because touch screens), overly padded out (because touch screens) and they hide all options behind a hamburger menu (because mobile screens are physically too small).

I'm certain there are theoretically ways to do it "right" (a friend web developer told me how but i forgot) but absolutely zero sites (that i visit) do that.

Responsiveness lets you target two platforms with shit UIs for the price of one good one*, which is what businesses want[0], per the good ol' "worse is better".

* - two for one up front, ongoing development and maintenance costs of modern responsive UIs is going to be much greater than doing things right 2 or 3 times, but those costs are beyond the next sprint, and pay the good chunk of salaries in this industry, so...

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[0] - And, unfortunately, this thinking became the zeitgeist of software development, so OSS projects do the same.