Some of the finest companies in the world always recruit from within and most often based only on potential and not prior experience for the role.
I worked for Unilever PLC for 11 years in the marketing department, regarded as one of the top 3 marketing departments in the world. I was promoted 5 times and on each occasion did not possess practical experience for the role I was promoted into, yet performed with distinction every time .... Because like you say I was always being groomed for additional responsibilities. There were/ are countless more examples like me in many other organisations.
The anti Giggs stance that he will not succeed as Manchester United manager right now is flawed. He has every chance of succeeding as as much chance as appointing an experienced pro like Moyes or LVG.
Given Mourinho's availability, I'd want the club to go for him, I'm a fan and I like him. But i'd be equally delighted if Giggs was appointed and think he has as much chance of succeeding as any of the other names being mentioned because of all the 'grooming' reasons already mentioned many times before.
You are right but you do what people often do, you ignore the context around you. Unilever is extremely well structured while United isn't, at United the manager is the structure. Also you have been promoted 5 times, your superiors didn't had the crazy idea of promoting you from your 1st position to your 5th without making a stop to the 2nd, 3rd and 4th positions.
It's possible that Giggs is ready, it's possible that Giggs is already brilliant but for everyone even the people around him his appointment will be like flipping a coin.