Why Did They Disappear? 

I have heard a lot of speculation.  One source claimed that the transformers burned out.  I purchased a Dareplane on ebay a while ago.   It was broken, and I fixed it.  I think I can speculate why these toys disappeared:

• It is possible that the transformers were bad; however, I would have to be convinced of this.  Transformers are really not prone to burning out, especially that quickly (days or weeks). 
• As I went about opening the control unit (the piece I felt most likely to be at fault), I found that it was glued shut.  It took me hours to finally cut and pry it open.
• Once inside I discovered that the motor speed control was actually a wire-wound resistor with a metal armature.  The armature had one lead of the transformer crimped to it.   It turns out that the crimp was loose and the wire had fallen off.  I reconnected it and it was fixed.

1. It seems to me that Mattel made two very bad mistakes:
   The connector to the internal speed control was crimped.  In addition, the crimp and wire were connected to a constantly moving part (the speed control lever).  It was only a matter of time before the crimp gave-way, or the wire simply broke.  Flex a wire back an forth for hours and hours, and see how long it takes to break it!   Yikes, what were they thinking!
2. The box that all of this stuff was housed in was glued shut.  That left no way of ever fixing it!  The only way to gain access to the control box was to pry or cut it open.  After that, there would be no way to ever sell it again.  Was this a cost saving measure?  If so it sure cost them.

I would guess that between these two problems it rendered the whole Dareplane line unprofitable.  I'll bet they just dumped the toys that were returned to them.

I'd also bet that fiasco influenced how their toys are made today!