It would completely ruin the game i'd imagine. It'd be impossible to enjoy it as it's intended once it becomes work. Once you know every design element, and movement. I know because I know a few game coders and developers. It applies to music too. When you make music, music stops becoming what is it meant to be. It becomes a job. It becomes something you critique, judge, tweak with, work on to improve. You listen to it to judge it and make it better, not to love it from the start. The rapper Joe Budden said it was one of his favorite parts about retiring. Is that he could listen to music with his fan ears again. When you are in the business, you listen to it as a artist and you see it as a project and not as a game. You have musicians who sing the song they wrote 3,000 times by the time they are done. 200 concert tours every year. Singing that song at every show. Every birthday party. After a while, they start to wish they never wrote it. It's engrained in their deepest nightmares. With game coding, you spend so much time working on the miniscule details of the same thing. I'm sure it gets old.
After thousands of hours sitting behind a desk, adjusting every single thing from minor puddles and lighting and dialouge. After it's done, i'm sure you want to be as distant from it as possible. At least for 5-10 years. Then maybe it will be fun to revisit. But it must really tire you out.
I think most of the fun devs get after creating a game is other people enjoying it. As an artist, that's how I feel about my art. It's fun to make, but the most enjoyment comes from what it makes others think or say. If it makes others happy it completes it in a way. Video Games are visual art in a way.