I definitely encourage writing and designing games even when you know that they're never going to go anywhere. It's like how it's still good to exercise and have fun shooting a basketball in your driveway even though you know that you're not going to make the NBA at the end of it. Plus, it can be really good practice for other projects that do end up going somewhere.
Just be careful of how big of a time sink designing a fake game can be. Even if you're planning around what you think would actually be reasonable for a dev team to make, the fact that you never have to judge a finished product and that you lack many other real constraints can let you get really carried away. I've lost a lot of time to Battlefront 3, Fallout 5, and Ace Combat 8 (I guess it'd be "9," now).