It's not anti-Giggs per se, it's more pro-United since we support the club, not individuals - no matter how storied they are. He doesn't have the credentials in terms of experience, or a track record of success for the job; and for some romantic reason - we've made a horrendous decision to 'groom' him on the fly - this at one of the top clubs in the world. The manager needs to install his own men without accommodating trainees at the behest of the higher ups (as is usually the case at other clubs - even Rijkaard didn't groom Pep; but the clowns in Madrid are grooming Zidane for sure), and tell you what - if say I'm the next manager, I don't want him anywhere near my staff because I don't want the manager-in-waiting breathing down my neck.
There are a fair few pro-Giggs arguments:
1. He was a legend - Has no bearing on his managerial credentials.
2. He knows the club inside out - Again, no bearing on managerial credentials, he might know more about the club than the average mug - but that doesn't lend itself to a managerial role.
3. He trained under Fergie - So did about a couple dozen current/ former managers - and look where they are.
4. He will learn from Moyes and Van Gaal's mistakes - What if he learns to repeat the same mistakes?
5. Guardiola - Seedorf, Inzaghi, Souness, Shearer, Gullit, etc.
6. He will stay for 20 years - No, he will stay is he's successful. Let's not put the cart before the horse.
Cons:
1. He has never managed a club before (4 inconsequential matches does not a manager make since our season was already over by all acounts - and he was under zero pressure).
2. He does not know how to manage in European competitions (big criteria for a club like United which should ideally have European ambitions).
3. He does not know how to manage the club when we're pushing for titles (and no, experience as a player doesn't count - Keegan had experience as a player, but he crumbled under pressure from Fergie).
4. He has lived in a sheltered environment with no experience outside United - that will hamper his development, the best managers are usually well traveled and it gives them exposure to newer things in newer environments.
5. We don't know how good of a tactician he is - can he outwit the likes of Simeone, Ancelotti, Pep in the big, big matches? Simply hollering - 'Wazza! tuck in' won't do.
6. The club is at a critical juncture - and he would represent a massive risk because there's no adequate sample size - and failure could set us back by a couple of seasons, and cost the club a hundred million if not more in terms of revenue - look what Moyes' tenure did. That money could be better spent elsewhere - like improving the squad...
7. Can he dissociate himself from cliques and loyalty attachments, and become a 'manager' rather than a buddy to some of the players? Seems pretty straightforward, but it's an incredibly hard thing to do for a beginner.
8. How will he react in the face of adversity as a manager? We have nothing to go by.
9. What if he fails? This is actually a pro-Giggs point. No other big club will take a flyer on him, and failing at United might shatter him. What if the divorce isn't by mutual consent, but acrimonious in nature? Wouldn't we be complicit in setting him up for failure by throwing him in the deep end when he's not ready?
etc.
If he becomes the next manager, that's ok too. Maybe he'll be a superlative manager, who knows? But it would be a very risky and unnecessary gamble that could blow up in our faces, and scar a legend of the club (probably for life - very few managers make a comeback after massive failure at a top club after their first job). He needs to do what Gary Neville did. Go out, manage another club, if you're good at your job - apply for the manager's position at United (if it's vacant).
People say he will never prove anything by managing mid-table clubs, but it could not be further from he truth. He could prove loads by getting them to overperform, maybe take them from midtable to Top 6 and qualify for Europe; then manage a slightly bigger club (if not in England, then maybe Spain, the Bundesliga or Serie A) - show genuine title ambitions, maybe win a Cup or two. And then he might be ready by merit. And he'd have done it the hard way so he'll value the experience - instead of being handed the position on a silver platter.
PS: And for the 1000000th time Guardiola was a once in a decade or two type of anomaly - not the rule of thumb. And he traveled to Argentina, Chile, Mexico, Brazil to prepare himself tactically - under Bielsa and the likes. Apart from learning from Sacchi's Milan, from Cruyff when he was a young player, from Van Gaal when he was successful etc etc. And that preparation is evident in the way he trains his teams - he's brilliant, crazy and exhaustive - which is why he can't manage for more than 3-4 years at a stretch and might burn himself out, or lose passion for the game. Pep is very very unique.
