Ask HN: How to justify side projects with a successful primary project?

Currently, I work for myself. It's a blessing and curse.

On the one hand I have more money and time than I ever could have as an engineer, but I find it hard to justify using my time on anything else.

How does one justify learning a new language, building toy side projects, messing with new technologies, etc?

I find it difficult to justify side projects when I could be expanding on my main project/money maker.

Ideas? Thoughts?