They actually didn't, because none of these were miniaturized PDPs in the sense they were compatible. As you say, they were variants, and they didn't actually r

They actually didn't, because none of these were miniaturized PDPs in the sense they were compatible. As you say, they were variants, and they didn't actually run the same software as the real PDP-8s or PDP-11s. Substantial modification was required and some OSes and application software was never ported. The DEC Pros in particular suffered from poorer performance as well (the 380 finally bettered this but was too little too late). If DEC had actually sold a compatible microcomputer PDP system, they might have done better commercially, but instead they used the PDP architecture to build related but incompatible systems so as to not cannibalize sales of their big iron. In that sense, they were successful, because they sold very poorly. I like my 380 with Venix, but it's a dead end.