Here's my problem with this assessment (sorry Evans, gonna have to disagree with your take here), Bucky isn't just the friend from youth. Bucky is Steve's friend, whereas Sam is Captain America's friend. Allow me to demonstrate from a set of gifs on tumblr that encapsulates the point:
(source: zendaya @ tumblr)
What unfortunately the first gif doesn't capture is that right before, Steve straight up asks Bucky, "Are you ready to follow Captain America into the jaws of death?" And Bucky's response was "Hell no." Followed by the line in the gif about following Steve Rogers into the jaws of death instead.
And that is the core difference between Sam and Bucky. For Bucky, Steve will always be Steve Rogers first, before he becomes Captain America. Whereas for Sam, Steve will always be Captain America first, before he is Steve Rogers.
Why is this important?
Because Steve Rogers and Captain America are not the same. Now, some might argue that Steve is Captain America, but that's just plainly not true. Captain America is a legacy character, a mantle that can be picked up by anyone. But Steve Rogers will only ever be Steve Rogers.
Captain America is a symbol of hope. He is good and kind and inspirational. He is someone who is utterly giving, unfailingly fair, and never without a kind word.
Steve Rogers, on the other hand, is a stubborn little punk who doesn't know when to back down from a fight. He's someone who doesn't like bullies and has a stubborn streak a mile wide. He has flaws and selfish desires and is prone to fits of jealousy and possessiveness. He is prone to making bad decisions when his temper gets the better of him. He's also not quite grown out of being a little mouthy kid from Brooklyn with a chip on his shoulder.
So Bucky is the one that sees the kid from Brooklyn with a chip on his shoulder and a stubborness that borders on foolishness, where Sam sees a national icon of all that is good, just, and fair. Sam puts Steve on a pedestal, whereas Bucky sees Steve as a human being.
Now, I'm not saying Sam won't come around to seeing Steve for the person he is under all the spangles, but the point is, for Sam, his first impression is always going to be that of a 6ft2 250lb supersoldier that single handedly saved 400 POWs. That will always be the first thing he thinks of when thinking of Steve Rogers. But for Bucky? No matter how big, how super, how ridiculously powerful Steve gets, Steve is going to forever remain that 5ft4 90lb asthmatic who refused to back down from a fight.
And that kind of first impression is going to forever color how they interact with Steve. And I think for Steve, to have one person who could see past all the supersoldier and into the person is something that's very, very important. Sam is no doubt someone he will always be indebted to, but Bucky is fucking family. It's not just about being friends for a long time, it's about how much Bucky sees into Steve that no one else will know, not without the kind of life time experience that Bucky has built up with Steve. Sure, give it another 20 years and maybe Sam will get to know Steve as well as Bucky, but right now? In the current continuity? No one knows Steve like Bucky knows Steve. No one will have the immediate knowledge of Steve and his stubborn little brain the way Bucky will know.
In short, Bucky's perspective of Steve is unique, a lens that no one else can have, and that perspective is, for Steve, irreplaceable.