This is a really great point and similar to an idea I considered a while ago when writing about a related concept. I wrote an article about why beginners should stop doing the daily ui challenge which had many of the same arguments you make here. I think re-designing a real product is a great alternative to daily UI.
I personally recommended in my article that designers create a side project and focus on that because it will force them to have a holistic approach to the user journey, user goals, product goals, etc. when designing — not just designing one screen that looks pretty.