If the iPhone mini was struggling at 6 million units sold per year.
If the iPhone Air is struggling at 5 million units sold per year.
If the Samsung Z Fold/Flip are struggling at 5 million units sold per year.
Then the iPhone Fold won't meet sales targets and become a success story either.
That is not how that works at all.
The iPhone Mini was the bottom of the flagship line, $700 ($600 after a year) iPhone that was identical in spec to the regular 12 or 13. It was given the exact same carrier discounts as the regular, the exact same storage and color options, same cameras, same everything. It’s only difference was the screen size, and, personally agree with it or not, people made their opinions pretty clear. Even though there was pretty much absolutely no compromises over the regular 12 (besides battery sorta) people simply didn’t care. People didn’t just prefer a bigger phone, but even more preferred a much bigger phone, seeing as the Pro Max was the most popular during that era.
And by the way, I had the mini, and loved it. I tried to convince other people to get the mini. I tried offering my Mini to other people after I was done with it. The response I got was pretty universal, nope, not interested in a tiny phone. Even if it’s the best tiny phone that’s ever been released, still not interested in a tiny phone. Literally would rather have a 750P LCD cracked screen XR with a heavily degraded battery than a Mini. Most people just didn’t want it and that’s it.
The fold will literally be in the exact opposite position. The most expensive device of the lineup, and also the most niche. And Apple is not dumb, they know this. But it being the top of the line, the most niche, and the most expensive also gives it a lot more wiggle room that the $600 Mini never had.
Its closest analog is like, the AirPods Max. Extremely expensive, absolutely not the best seller, rarely updated, but it has its niche and its price is high enough to continue to sustain it.