EDIT:^Spidey coming in with the clutch facts.
Of course it's not supported yet, because we haven't seen Bucky become Cap yet. But as I say, it's been foreshadowed it every film Bucky has been in. It happens in the comics. And we know Chris Evans will be done after Avengers 4, while Sebastian Stan will still have movie left on his MCU contract.
Do you really think that would be a effective ending? Or would it be more effective to follow Ed Brubacker's comics - which the Cap movies have been hugely influenced by -and have Bucky carry on the legacy of Cap himself?
They were able to foreshadow Bucky having the shield even though it hasn't happened yet, they could just as easily foreshadow him having some other attributes of Captain America, but nothing other than having the shield has been foreshadowed, because nothing else is important for his arc.
Him having more movies left is interesting, but that's a far cry from MCU White Wolf being 616 BuckyCap, because they are different characters important ways. MCU's White Wolf doesn't live in a world of legacy characters, neither can he share an organization with Tony Stark because he killed his parents. There's no doubt Brubaker's run is important, but important is not the same as canon. Crucial divergences have happened, and these trends away from BuckyCap are only likely to grow.
EDIT: If we're going with foreshadowing, every movie Sam Wilson has been in, he's foreshadowed that he's going to keep doing Captain America-type things.
"Don't Look at me, I do what he does only slower."
"people who shoot at you usually wind up shooting at me too."
Defending the Avengers base in Ant-Man, recruiting Ant-Man.
Laying on the wire (and leading others to) for Cap and Bucky to escape
If we were to pick a character whose arc could most suitably lead into Captain America's *role* in the Avengers in terms of leading tactically, inspiring and just doing the darndest rightest thing with no pretense, who would that be? Because it ain't Bucky Barnes.
Consider for a moment that maybe... *just* maybe these two characters are being set up to be a duo.