"I found that using only one monitor allows me to focus more on what I’m doing, and I don’t miss anything about a multi-monitor setup."

This.

I had the same realization maybe 7 or 8 years ago and it is true for me. With multiple monitors it's like moving your hands away from the keyboard all the time and looking for the mouse. Same happens for me while looking for anything across the 2 or 3 monitor setup. It just weakens your focus... If I'm on a laptop (and I usually am), I just use the laptop's screen. Getting used to a different screen size while working with my laptop setup would also slightly slow me down when I'm bound to use only the laptop.

Except you have huge monitor and you lay stuff in multiple side by side windows which makes the whole argument very moot. And when you use laptop you are kind of forced to use single screen because multiple screens is inconvenient at best.

When I started development 15" monitors were standard and 17" were a luxury. Setting multiple monitors was a must or you would have to constantly switch between editor, terminal and documentation.

If you look at the setup that's exactly what is happening. On a single screen there is browser with documentation (presumably), and there is four (!) IDE/editor/terminal windows.

It's not about how many monitors you have but rather what you are doing with them. You can put Netflix as one of the windows and still get distracted on a single monitor setup or you can have bad sight like me and require two 27" monitors side by side so that I can magnify fonts and have documentation, IDE and terminal at the same time with no strain.

I have three monitors, but I only use one (a 24" one, the second is for when I want to watch some show or movie while doing stuff on the main one, and the third I basically never use). I have my apps always maximized/fullscreen in the main monitor and I alt-tab between them. I never could get into having multiple apps on-screen at the same time, since I can only focus on one.

Writing this comment, though, I realize that a better alt-tab switcher would be a godsend. I keep getting confused with window order sometimes and I'm not sure why, maybe I should write a better switcher.

A better alt+tab switcher would be great! Why isn't that a thing? I'm imagining something where you could use alt+f1, alt+f2, alt+f3, etc. to switch to specific windows rather than just rotating between everything.

On Mac, Quicksilver with Triggers: https://qsapp.com/