From a strictly United "played under Fergie" model - we have the likes of Robson, Bruce, Strachan, Solskjaer, Keane, Hughes, Ince, Blanc etc as examples. Not exactly a glimmering list of success. Given such mediocrity, why on earth would Giggs be any different ?
This relates back to one of the earliest discussions I had in this thread, which can basically be summised as "Some managers will perform better with a big team than a small one."
Now dont get me wrong, I am not saying that all of those names that you listed are secretly great managers who were just never given their big break by a top club, but rather that when you talk about ex-United players, we are talking about players who were immersed in a club of United's stature (not in an arrogant "we are better" way). What I mean by this is that they were used to the strict regimes, the setup and the environment of a highly successful, title winning club. I think it is fair to say that as managers they would probably try to implement something similar at their clubs, and therein is the (potential) problem. Trying to coach a Sunderland to be title winners is probably a bit of a lost cause. The environment, the expectation and the job itself is very different.
Following on from that, it is for this reason that I believe that if Giggs went and spent 5 years at Aston Villa and was a great success, it would not prepare him or qualify him at all for the United job. Similarly, if Giggs went to Aston Villa and didnt perform particularly well, I wouldnt see that as a "See, its lucky we didnt give him the United job" situation - the scenarios are just too different to be comparable. He might as well go and manage a supermarket.
Fair enough. But again, like with some of the other alternatives, there are loads of presumptions involved here in terms of the longevity conjecture, evidenced by the ifs and highly possible arguments. The narrative of him continuing for 20 years sounds awesome, but surely you must agree with the fact that it's far from certain, instead of being highly possible? Who's to say Giggs won't turn out to be a one and done man like Johan Cruyff. He too had a deep history with Barcelona, he helped found La Masia in its current guise, he set the template for a system that survives to this day, he build the dream team as they were called, with Romario, Koeman, Stoichkov, Guardiola, Laudrup. And who's to say an outsider like Pep doesn't fall in love with the club, and decides to stay? Arsene Wenger for one isn't British, and one might've guessed he would have left Arsenal long ago, given the interest from Real Madrid among others. Yet here we are, and he's the second longest tenured manager in Premier League history after Fergie. I personally wouldn't call a 20 year stint highly possible when it's fraught with a million variables. And why don't the other candidates offer that in absolute terms? When as mentioned above, Wenger never evidenced the desire to drop deep roots at his previous club, yet he did so at Arsenal.
I get your point here, but we are just talking about probability. Of course there is no guarantee that even if he were a great success, Giggs would stick around for decade after decade. What we can say however is that he came up through the youth system, he loves the club and wouldnt have any particular reason to move on.
With regard to your "what if Guardiola fell in love with the club?" point - again, its just about probability. Guardiola was brought up at Barca and managed a great team there - I just cant see why he would fall in love with United and stay here for a longer time than he did at his boyhood club, where he was highly successful anyway.
I take your point though, that it isnt as simple as "If Giggs does well, he is here for life." - but he certainly has a higher chance of that than an external candidate I think it is fair to say.
And honestly, I don't like the arguments about knowing the club in the slightest. What does that even imply, does it take decades to learn about how a club functions? Are Pep and Ancelotti idiots who can't fathom how work their way around things? Or about its decade long history and association? Eric Cantona for one is a passionate United man through and through, and he was here for just 5 seasons, with no previous links whatsoever. Is he any less of United man? Great managers can grasp things on the fly. I'm pretty sure someone like a Guardiola would immerse himself from the get go, and know everything there is to know about the club in a couple of seasons at max. So is Giggs' continued presence that big of an advantage? And while Pep, Ancelotti and Mourinho have chinks in their CV, they are far, far superior to Giggs when it comes to their overall resume, aside from the tangential stuff like understanding the club and whatnot. What's so special about the United situation really, in objective terms?
When I talk about understanding the club, I more mean understanding what it is to be the manager at United. The responsibilities, the expectations, the day to day managerial duties etc. He has seen Ferguson and LVG doing it, and has had a crash course in "what not to do" under Moyes. Whether Giggs can make the step up and replicate that sort of success is unknown, but he has been given the tools and the knowledge to maximise his chances of doing so, and those inside the club seem to believe that he can.
This is less a "Giggs has this advantage over xyz" argument as rather that that knowledge and understanding (in my opinion) helps to make up for his lack of experience. You mention "is it about commanding respect" for instance - the point is that Giggs would indeed command respect at United, regardless of his inexperience.
How is Giggs only slightly more risky when compared with managers like that, who have proven it at the highest level possible, and Giggs is just an assistant manager with a managerial record spanning 4 caretaker games? The United managerial post is just another top, top job from an objective standpoint, we're not super special. Every big club has traditions, every big club has expectations, every big club has a great history, and those managers are well equipped to deal with that.
Dont get me wrong here, if we appoint Guardiola after LVG then I am not going to be putting myself on suicide watch or standing outside Old Trafford on a soapbox preaching that the end days are nigh. My view is that LVG was only ever a short term solution, and that his role is to prepare the team and the club itself for the next person - which I think will be Giggs. I think that if LVG's tenure is relatively successful then the club will consider appointing Giggs as the best option for a seamless transition, continuity etc, as someone who can take the team and philosophy that LVG has implemented, and take that to the next level in the same way that Guardiola did at Barca.
As
@Roboc7 pointed out - if we were planning on appointing one of the touted names, why didnt we just go for Klopp or Ancelotti this summer? Why dont we approach Guardiola now? Is there really a major benefit to letting LVG see out his contract if we are then going to just bring someone else in who will start afresh and rebuild it all again anyway?
Fair play, Giggs is an unknown, any projection made with him is an highly inaccurate one. The last two times we acted like proper idiots, obviously we could do it a third time, United try to much to be cute with Moyes, LVG and now Giggs.