Steam Sales have a massive effect on games - thousands of new players try the game simply because it happens to be 15% off today or whatever. Even the "Free Weekend" play converts thousands of players that find the game fun. It makes trying out a new indie-type game feel less like a risk.
Now that Factorio has real competition in this type of building game (Satisfactory - which I highly recommend if you liked Factorio), they might discover their principled stance against any Sale might waver a bit.
To each their own I suppose. One day, Factorio will be on sale... once the playerbase dries up and they desperately try to inject life into the game. Or not... maybe they let it wilt away... which would be a real shame.