The 80186 was a market failure because, mostly, PCs built with it were not completely compatible with software already existing. This was because the components the 186 integrated were not copies of their IBM PC analogs. IIRC, at that time, the "gold standard" of PC-compatibility was Flight Simulator: if anything was even slightly different, it wouldn't run. Many PC almost-compatibles had special versions of it.

Intel learned their lesson and all subsequent processors were crafted to be exquisitely compatible with all processors that came before them and to make it easy to build computers that ran all previous PC software. AMD64 (the architecture) is limited by the need to run x86 code seamlessly. Everything is there, from string copy instructions , x87, MMX, SSEx, either explicitly or as parts of newer standards that superseded them. Until recently, every PC chipset had, deep in it, a fully functional ISA bus.