I think if you mainly do writing, an iPad with an external keyboard is a viable solution. Depending on what software you use to write. Once you start using a “retina” display, though, you probably won’t be able to use your old Macbook Air again. I have a old 20" Apple monitor that a designer friend graciously gave me when he upgraded a several years ago. Once I got my retina MacBook Pro in 2014, I couldn’t bear to use the 20" monitor anymore. The fuzzy text on the 20" monitor is unacceptable now. LOL!
I need Mac OS for the music software and music notation software I use daily. And I prefer the Mac OS version of Scrivener, which I use for all my writing, but I love my iPADs. I have a 9" that I use for general web browsing, reading -including Medium, watching videos and Netflix, and playing a few games. And I have a 12" that I use mainly to display music when I am gigging (the big screen is SO nice for that) and to make comics.
But I mainly write on my laptop. I don’t have an external keyboard for my iPad or I might consider using it for writing. I do short responses to Medium stories on my iPad with the onscreen keyboard, but not anything long.