Can a tech journalist and founder of The Outline use iPad Pro as their main machine? Probably not but it is possible (see Federico Viticci). For many people that is not the case. Applications like Logic Pro, Final Cut Pro and Photoshop do not have iOS equivalents as much as people would like that to be the case.
To say that everything "works worse than you expect" is a complete over exaggeration and in fact the opposite is the case for many scenarios. E-mail, web browsing, YouTube are all better on iPad. Many apps are just better on the iPad platform. For example, I'm writing this article using Drafts. There's no Mac version of the app and doing any longform writing on iPhone would not be pleasant experience. I prefer to write and research on iPad as opposed to any other device. It's the clear winner in that situation for me.
As the years go by I find myself using iPad more and more for work. I can't imagine not having it as one of my tools. I haven't felt the need to upgrade my 2012 Macbook Air but I'm itching to get the latest iPad Pro. There's just more interesting and innovative work happening on iOS in 2017.
This is not to diminish the Mac or any other traditional desktop/laptop. They have their purpose just like iPad. I do fear for people that do not embrace the future of computing which I do believe includes iPad and iOS. While there have been some hiccups along the way, direct on screen manipulation feels more like the future than a mouse and cursor ever will.
Even if you're a "Windows" person, Microsoft has completely embraced touch and pen input at this point. Their version is completely opposite to Apple's as they implemented touch on desktop software as opposed to a touch-first operating system. Time will tell which was the correct choice but for many iPad is the future. Denying this is completely confusing and irrational.