The only question that matters with any change is: did it work?
Everything else is supposition.
If the change was made to secure our survival, it unquestionably worked. Could Dyche have kept us up? He might well have - but if we were told in January that Moyes definitely would, we would have made the change whether we still believed Dyche would or not.
It was the same with the Allardyce appointment. People claim to this day we would have stayed up anyway, but that's immaterial. Allardyce had us safe within a month. Unsworth and Joe were getting trounced at Southampton. Nobody was waiting around to see if things got better... They might very well have (and we did beat West Ham, albeit with Big Sam in the stands bringing a new, er, vibe to proceedings), but when you get to that point people want security - not hope.
The Moyes appointment was an unequivocal success - if the goal was survival. When the goal changes - as it does now - then the jury, as with all managers, will remain out.