
If the goal is to publish on Steam to earn some money, at what point during development do you seriously have to consider "will my idea actually be worth publishing"?
If that's your goal then that has to be the focus from day 1. Picking the right game idea is absolutely crucial if you want to find success. You have to pick the right genre right from the start. You cannot really start from an unmarketable idea and make it marketable afterwards.
Then once you have what you think is a solid idea, you need to validate it. That's when you make a prototype or some gifs, post them on Reddit / Twitter to get some playtesters, and try to figure out whether the idea has potential, or whether you should go back to the drawing board.
If you get absolutely no traction then it might be wise to kill the project right then and move on to a new idea as opposed to trying to salvage and idea that apparently players don't seem to want.
But the most important thing is being very clear about your goal. If you're doing it for fun then do whatever you want, but if the goal is financial success then you need to treat it very seriously right from the start. Treat it like a professional, like serious job, because it is.
Best of luck!

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