Meanwhile applications like skype, other instant messengers, slack, music players have grown and now are fullscreen by default. Non-blog websites are usually large and can't be displayed in a simple window. People are complaining about 80 character rule for code, and go to 120 characters and beyond - which again means you can fit fewer windows on a screen. I think web browsers and websites are largely to blame. Because that's like most users interact with computers today, that's what they expect and don't know it can be any other way.
Every single application wants to be THE fullscreen application. I think it's an admission of defeat! Over the decades, they've tried - and failed - to make smaller application windows that people consider useful. And it's not the fault of application makers - it's the broken "desktop metaphor" where you're supposed to move windows like physical objects. It works on a desk because you have two hands and 10 fingers. Imagine working at a desk (no computer) using only 1 finger! That's how it feels using mouse. The default window managers are crap at actually managing windows and arranging them usefully. Dragging corners, window borders, moving windows feels miserable in the long run, and when you close one of your windows you need to repeat it when you want another app window to fit into your layout. So many people just don't bother, get a bunch of fullscreen windows and alt-tab through them.
And applications with tabs are a symptom of the disease, too. Web browsers, the blue Microsoft Word, IDEs, and so on. It's alt-tab fullscreen windows in sheep's clothing. Nothing particularly wrong with alt-tab method, but it doesn't scale to a large number of windows we have nowadays.