Thanks for you feedback. Indeed, I spent more time making conveyors work properly (6 days) than balancing the game (1h...), and ~1 day making machine, consumer, generators, walls, (bad, mmmmh silicon) graphics. (the rest of the time, I tried making a runner game but I dropped this idea because I hate physics...). 

In my mind, the player should have pay a rent for every tile (so the player is forced to create a running factory to pay this rent) and every block should have a maintenance cost, as well an electricity cost or things like that. I had other ideas where the player should manage contracts with producers and consumers competing with other fictitious factories. In brief, many things to prevent the player to just produce one item and wait.