Yamaha's audio styles are in response to the Ketron Audya in my opinion.
If Ketron were any significant competition to Yamaha, I'd say you were probably right. But the audio feature on Ketron's hasn't equated to a massive spike in sales, and the other manufacturers don't seem to be losing significant market share to them.
Personally, I'm more of the opinion that audio has been added to Yamaha's because it is EASY... The code for audio manipulation is well matured, pretty bombproof (once upon a time, tempo and pitch manipulation of audio resulted in all kinds of nasty artifacts), and engineers versed it its use are commonplace.
Mind you, I said adding the audio features was EASY.... I didn't say it was easy to turn it into something PRACTICAL! Yamaha's engineers obviously found it easy to add the core audio functionality, but apparently found it hard to add the features that would make it of any real use...
And this, finally, cements why I think Ketron had no influence at all Yamaha's decision to go audio drums. Let's face it, if Yamaha had spent any time with a Ketron, they might have actually got it RIGHT! They have ignored every single audio feature on Ketron's that makes it a practical feature -
Most of the styles being audio (so you aren't forced to use MIDI styles most of the time), easy import of your own audio into a style, fast, practical loading of new audio styles, and looping capability on the audio.
Yamaha may not even have heard of Ketron (they big in Japan?)!