DeX is perfect for certain business settings (mostly sales, but many management functions also) where you home-base out of a desk, then hit the road during the day.
When you home-base, you're producing more in terms of text, longer form emails, documents, spreadsheets, presentations etc. So having a keyboard+monitor+mouse is important.
When you are on the road, you really just need to make calls, text, get driving directions, send short emails, etc. Occasionally make a presentation, which is doable off of the phone without any external devices.
I've done this workflow in short spurts and it's frankly really fantastic, modern productivity tools usually have an Android app version that's fine, or a web-app version that's also fine. I was often also making calls from the same phone that was driving my KVM.
Pretty much the only thing preventing me doing it permanently is multi-monitor support at "homebase", and sometimes being able to print sanely on a corporate printer setup.