I though “failure” was an absolute term? They obviously aren’t done developing yet, but that doesn’t mean they never will.
I bet they have working prototypes and it comes down to something like power draw being too high. They probably want something that’s at least on par with what Qualcomm has.
And even when the first “retail” version is done, I find it highly unlikely that they put it in the flagship iPhone first. The modem having a bug or other weird behavior in their most popular product would be detrimental. They’ll test it in cellular iPads, or maybe even in MacBooks. If they test it on an iPhone, it’ll probably be on the iPhone SE first.
And after all that, they’ll put it in their flagship iPhones.