Let's just clarify what I said - which was that diagnosis of and recovery from a stall is something that *every* pilot *should* know and understand.
I also said that an HUD was not necessary for stall detection and recovery - I certainly did not say that an HUD would not be useful under certain circumstances. But in every circumstance I can think of an HUD falls definitively in the "nice to have" category rather than "essential". This is very different from a fighter specification where the likelihood of other fighters shooting at you means that going head-down on instruments can sometimes be a very bad idea.
I believe that HUD remains an option on airliners from both sides of the pond, but, possibly due to the expense and extra complexity involved, take-up has not been universal.