But what if neither caravan brings an anvil? Or if he went into a mood sooner?
I've had close situations to that before, only difference being an anvil at the start.
a) The chance of the first caravan not having at least some kind of anvil is low to begin with.
b) i believe there is a 20 dwarf limit for moods, which means you dont have a chance for it until the first migration wave, and even after that you have to have 20+ dwarves, which needs a 14+ wave. Not sure how it works for you, but i usually only get a smaller wave in late autumn, or a bigger one in winter / spring. Usually the earliest a mood can happen is spring next year (as it happened with me)
c) even if a mood happens, it is chance based which dwarf gets the mood. with 20+ dwarves and only 3 that have metal related skills you want a mood for, chances are not particularly high that the proper dwarv gets chosen.
d) even if you get the mood, you might still be able to salvage the situation if the humans come in summer second year and bring an anvil, which they usually also do.
Bottomline:
Not bringing an anvil might cause some problem, but for that problem to occur, there needs to be a series of rare random things happening at once, making the odds of actually encountering such a problem extremely low.
Its pretty much risk vs reward
You risk a little by not bringing the anvil in exchange for more flexibility on choosing your embark items and skills.
You probably forgot a bin or two the first time, or deselected some of the things you were going to take.
"You offer us 5,000¤ in earrings and bracelets for our 4,000¤ of silk, cheese, booze and anvils? We spit on your pathetic offer for our luxurious goods. There is another fortress just on the other side of the mountain that will give us far more for goods of this quality."
"How about we throw in 50¤ of bins as well?"
"Wonderful, absolutely exquisite. Yes, 5050¤ is sooooo much better than 5000¤ that I feel ecstatic about the offer now."
Just doesn't happen. But if you take out a few of the more expensive items that you were wanting to buy (bins of GCS silk, dwarf cheese, armour, bolts, anvils) to decrease the value of goods you're buying, and optionally add in the bins to increase the value of the goods you're giving away, THAT can turn a rejection into an ecstatic acceptance. So there's something you're neglecting to mention, or just something that you neglected to notice...but what you're describing absolutely will not happen in the game, end of story.
I guess i will make some tests the next time i have another massive stock of rock junk for trading. Until then, i accept you are convinced you are right and i still believe that i saw what i saw