The Curl i4 and Ei4 are the latest I know with IGH. Before that, there were the Curl i8 and i7 also with IGH.

The fork and brakes and seatpost changed several times too, some version had a kind of folding parallelogram saddle mount. There was a version with mounting points for a Brompton block.

If I remember well, the Curl appeared in 2015, i.e. 10 years from now!?

And as said, the issue of this bike is not lack of commercial success, its lack of commercial availability.

Why did Dahon invest in the development of a bike that wasn't really commercialized, wasn't helping to sell other Dahon bikes, wasn't a flagship showing the expertise of Dahon (when you read the press releases and reviews of the Curl, most of the time its presented as a Brompton clone and not as what it is: a really innovative competitor to the Brompton)?