Wow, just wow.
Alright then. If anything works on Prince, I would like for you to try to win with these conditions:
Never build military units.
Never befriend City States.
Research every tech in every era before advancing.
Never adjust city output.
Do you really, genuinely, not understand the point we're making? Prince is so easy that you can win without doing things that are actually good strategies, or that will work on higher difficulty levels, or are actually sensible. So its not a useful benchmark for discussing strategy or balance.
If you don't make a practice of killing every AI on your continent with (insert favorite rush tactic here), then it is perfectly possible to end up with Gunpowder and no Iron
Only if you make a deliberate lack of effort to try to acquire iron (from settling new cities, conquest, or city state alliances).
Yes, if you try hard to play badly, you can end up without iron, but if you are trying to acquire iron, I have never seen it be a problem by the time you have gunpowder tech. (It can be a problem before then).
Even then, Musketeers are stronger and cheaper than Longswordsmen.
You're seriously going to make an argument about the value of a unit by talking about how good its UU replacement is?
I have been in games where I had a significantly number of both Minutemen and Musketeers. They're quite good.
The question is not: are minutemen useless. The question is: are the underpowered given that they take up a precious UU slot? I think there is general agreement that America is the weakest civ.
I don't think musketmen are underpowered stat-wise, like I said, I think their problem that they fail to achieve their design intention (spammable medium-power unit) because they are dominated by longswordsmen and knights, which are too readily available because the strategic resources aren't actually strategic. And because its too easy to use great scientists to rapidly beeline to rifling.
If using Musketmen is "something reasonable," that "will work on Prince," then clearly, it's not a useless unit, no?
The question is never whether something is completely useless, its whether it is underpowered or not.
What is happening is just plain old elitism, that's what.
Oh noes! People who are good at the game and understand the strategy engine are driving the discussion on balance and mechanics! Quick, lets balance the game around the people who don't understand how to play it very well yet!
Why is it a bad decision to make Musketmen?
Because they're underpowered relative to building longswordsmen or knights.
Just because you can win with them doesn't mean that balance is ok.
We could weaken them further and you could still win with them, but that wouldn't mean that its a good idea to weaken them.
Balance is about making interesting strategic decisions where multiple options are equally effective, not about having one more effective option and another less effective option.