Strong same, I've been working on a 14" laptop for the past 2 years with no external monitor or keyboard. A second monitor makes me twice as productive at procrastinating.

Given I can only reasonably have a single application open (which is an IDE with full documentation/intellisense inside) it means I'm forced to focus, apart from right now obviously.

Having a lot of monitors might encourage multitasking beyond what one's attention span can handle. If you have 4 monitors, you'll be tempted to throw YouTube on one and news sites on the others. A single monitor forces you to replace your code editor with YouTube, and people are probably more hesitant to do this.

At least a second monitor tends to be super useful, especially if you ever need to refer to documentation or another file while coding.

To add, while this study isn't measuring actual productivity, people say they expect/feel like their productivity increased 40% with a second monitor:

https://www.jonpeddie.com/press-releases/jon-peddie-research...

same. i've got 3 old monitors that just keep working. could I get a new widescreen to replace all three? Yeah. will I? Probably not.