I am afraid you are ill-informed. Long-term availability is a crucial concern for many industrial and embedded applications so numerous companies sell x86 mainboards with a guarantueed minimum production life of 5 years.
For example, Kontron sell Mini-ITX mainboards that are compatible with Intel i3, i5, and i7 processors and they advertize up to 7 years of availability after release. These type of mainboards are very similar to consumer mainboards but generally use better components and are specifically designed to last.
It should be noted, though, that a quick Google search suggests these boards start at about £280 including VAT, much more than an equivalently specced "normal" motherboard.
If A-Eon or Amigakit were to use these, they'd have people complaining about having to use years-old x86 motherboards when there are much cheaper brand-new ones out there.
Either way, they can't please everybody, so better to do what they are doing I think - make a design decision (i.e. custom PPC motherboards) and stick to it. Better than to keep changing direction.