The problem is that just about everything short of modeling VSTi sax programs use samples... which are a snapshot of a sound. But you put a horn to your lips, and every single note is, or can be, utterly different to every other note.

If you want a growl sax, you have to switch to a growl sax patch (if you have one), but a real saxophonist can start a note smooth and sweet, make it brighter and rougher as he plays louder (still the same note!) and end up with a growl and a squeak! And that's just the first note he plays!

A sample is a recording of a note. Played one way. Too bad if you want to morph it into something else!

The thing about the breath controller when used with the DX-7 (or other FM based synths) was that the breath control was used to not only get you amplitude control (in other words, when you played a note, nothing came out until you blew) but that the strength of the blowing radically changed the tone of the note. And so, by tonguing harder or softer, the attack of the note changed, not just in volume, but timbre too. And, as you continued to blow, you could blow harder or softer to get the timbre to change.

This, combined with volume pedal use, allowed you to do things like blow hard and reduce the volume, to get a quiet but bright sound, to blowing softer but raising the volume, for a full volume, but mellow sound. Or hit the note hard and die away, then swell back. You simply cannot get that degree of control over sampled notes.

So, it's not the problem with the breath controller, it's a problem with the keyboard it is controlling.

FM, while it had its shortcomings (it was a bear to edit and create new sounds on), had all kinds of advantages over samples. Not just with the breath controller, either. The famous DX-7 electric piano sound was so popular because it was so expressive. If you listen to most sampled E. Pianos, you'll hear a sudden change in timbre as you play increasingly harder, as different samples (recorded at stronger and stronger playing levels) get switched to. But the DX-7 went from smooth and mellow to bright and bell-like completely without any jumps in timbre, simply a gradual smooth change.

Until you get into GigaSampler sized E. Piano libraries, with so many different samples per note that the change in timbre becomes smooth again, all WS and arranger E. Pianos pale in comparison to how smooth the timbre changes were on FM synths.