Yeah, spot on.
Bruce, Robson, Hughes, Strachan, Giggs, Solskjaer, Stam, Blanc, even Phil Neville may be seen as just mediocre managers, but they are all highly successful managers in the grand scheme of things. Anyone that manages Internationally or regularly in the highest league division in their country has to be classed as a successful manager!
As you say, it's the elite managers that are a rarity and there are probably only about 10 in the whole world.
The likes of Gary Neville and Scholes are the only real 'failed' managers.
Ince and Keane are strange ones. They were good enough to be given the opportunity, but toxic enough for no one else to want them! I'd probably have them as failures too, but not in the same category as G. Nev and Scholes.
There's also a list of successful assistants/coaches from the Fergie years; McClair, Anderson, Phelan, Carrick, Butt and van Nistelrooy, amongst others.
As a percentage of players that become managers or continue to work in football, it has to be very high in comparison to other famous managerial eras.