That being said, I realize that some apps and intranet sites will be Desktop only by design, and that's fine. We don't want to make your experience worse. Any feedback on that front is greatly appreciated.
For example I see far more job adverts for internal tools developers, which are going to be by and large desktop only, than mobile-first website developers.
So I'd have thought the number of desktop only web app developers vastly outnumber the number of mobile-first web developers. I think you're wrong with the word 'some'. Most is the word you should be looking for.
Don't forget you're emulating a phone without touch. It's useful, but it's only part of a mobile testing process. So you're making what is a fairly broken process the 'focus', which seems pretty odd.
Mobile is also feature poor compared to desktop so generally needs less testing. It's the less intense part of any site because of the form factor. That's why you're not playing Fallout 4 on your mobile, you've got a little companion app for it instead.
Finally, for most mobile websites there will be a desktop only admin section. And it's desktop only because it has huge tables, with lots of sortable columns. On multiple pages. Common admin interfaces found in any website or app. So while a desktop site can often exist without a mobile component, it's rare for a website to exist without a desktop component.
For example, my phone has a resolution of 1080x1920 and a dPR of 3, so the viewport size is 360x640.
If I set these properties in Chrome Dev Tools, I just get a 360x640 rendering out of it. I'd like to be able to get a proper 1080x1920 rendition of the device screen too! It doesn't matter that it will look absolutely enormous on my desktop screen, I just want to make sure that my code and styling actually works like I want on it. I really shouldn't need to use an actual device for something as simple as this.