@Mr H I think it's a bit unfair to use the example of Pep and Zidane because of the squad's they had at their disposal - not much rebuilding was required. Their clubs could take the risk, because failure was most likely not going to cost them UCL qualification, at worst. But I agree with your general point that no one knows how Giggs will fare as a manager, until given the chance/opportunity.
And if past events are true, we did actually have a succession plan of sorts, which involved handing over the reigns to Giggs (if LvG was to be believed), after LvG completed his vision and rebuild. The moment that plan didn't come to fruition and another year of not qualifying for the UCL was going to start becoming extremely costly, Jose was brought in and it's safe to say, the entire succession thinking involving Giggs was abandoned.
I'm not sure if I'm actually arguing for or against Giggs getting a EPL job
but the progress for managers isn't always linnear - in any line of business. Just because one manager had to start out right from the bottom, doesn't mean all managers have to follow the same path. Fact is, how much weight do we put on the "experience" of Giggs working for over 2 decades in the presence of someone like SAF versus a manager who's done well in the Championship for a year or 2? Not to mention his shorter experiences under Moyes ("how not to be a football coach: for dummies") and LvG.
Obviously it's a massive risk appointing someone with more limited, or next to no, managerial experience, versus someone who does have experience (say Dyche or Howe). But then again, how much weight do we put down to the legend of Giggs and the respect that would command in a dressing room? It's not just any no-mark football personality.
Bottom line though seems to be that in the world of ever increasing revenues in football, the cost of failure for clubs/owners is massive. So if someone is willing to take a risk on Giggs with limited to no managerial experience, fair play to them. But if Giggs is also serious about his managerial career, and he realises that clubs are less willing to take such risks, he should maybe contemplate taking over at a smaller club/lower division in order to "earn" his shot.