Beckham was a great right midfielder with one of the most accurate right foots ever. His position I believe is extinct in modern game. You are either a winger or wing-back in that area nowadays.
What backtracked his career was the brand building, something Sir Alex opposed and warned and eventually it ended their sporting relationship.
Bellingham is still a kid. He's walking in the steps of Beckham with the brand. Time will tell.
I think the thing about Beckham is he was never as great as his fanboys think, but hardcore football fans underrate him too by thinking he was just a celebrity.
He's an attention-seeking imbecile, but as you say he was a very good player with a great right foot (though he tried to showcase that with far too many Hollywood balls imo). Some of his passes went astray, but from 1998-2001 at Man United was the best part of his career. He was excellent then and performed better than the more lauded (now) Giggs and Scholes. Beckham and Keane were the best of the four midfielders in the treble season.
I agree too that his position is basically extinct because he was a classic winger in a 4-4-2 (I never believed he could play CM). He didn't have the pace or trickery to play as a left wing forward in this era, and most teams don't play with a classic right midfielder.
Ferguson did indeed anticipate Beckham getting too big for his boots with the PR. His branding is very repulsive, in his prime he was like a much better looking, less gifted Cristiano Ronaldo with the branding and you'd think approaching 50 he'd give the attention-seeking a rest. However, he was a quality player - overrated by fanboys and underrated by those who dislike him, I feel.
As for Bellingham, I totally agree - as someone else said he has no discipline and is an individual who appears to fully believe his own hype. That could be the worst thing for his career. He has talent but I feel it is wildly overstated - he's fast, strong, has a decent touch, but he's more physical than technical - he doesn't see the full picture of the pitch, he doesn't think two steps ahead like a Zidane did, he's prone to drifting in and out of games.
I just don't think he's as good as claimed to be, and I believe some reasons why he is so lauded are his nationality, his 'physical' style (not saying he wholly lacks technique, but for some reason people prefer these physical type of players to diminutive technicians), the rush in this era of social media to claim any promising young player is the new Pele, and his arrogance (many people identify with cuntish personality types unfortunately, see Ronaldo).