(Author here.) It's a thing I am trying to do right now: 50% for data science freelancing (which I enjoy a lot nonetheless), and 50% for my own musings and self-development, without any restrictions (whether technical, scientific or educational).
It's clearly better financially than academic jobs _and_ I am not restricted by grant topics & timelines or a place.
I am just curious what can go wrong? (I.e. I don't know personally anyone living this way.)