Yeah, I think it's hard enough as it is. I prefer setting some ground rules, like making an ethically moral fortress, which already makes the game difficult. Those who want to play by the game's rules (especially in the current alpha version) will find it a bit too easy.. but they can just get all the tons of difficulty mods out there.
IMO, a game should just be easy enough for the average player to get through. I mean, heck, this game is all about dying, and I'm sure there's going to be more fun deaths when things like poison and dwarven smallpox and vampires and titan mages are fully implemented. After that point, there'd be plenty of mods to make the game more Fun, if it's too easy.
Though, there is a point to be made about the game getting easier as time passes on. Enemies don't become stronger, but once you establish yourself as a major power, a fortress-state would end up having foreign powers ally to destroy them, or suffer from internal politics ripping it apart, excess luxury and complacency, and dangerous ideologies like capitalism which would lead to some capitalistic spiral where only 1% of dwarves hold 80% of the wealth of the fortress.