What's hilarious to me is there was a time when the Avengers properties as a whole, including Iron Man, were considered past their expiration point. Before the first Iron Man movie came out Iron Man was not a popular character AT ALL in the comics. In fact, because of that, in the early 2000's, where Mark Millar claims that Marvel gave him pretty much given carte blanche to do anything he wanted with the Avengers (and we all suffered for it... sorry Millar fans, am not a fan of his) because at the time the X-Men were the more popular franchise and Marvel didn't really care what happened to the other heroes.
And after Millar's Civil War came out, the comics community pretty much hated Iron Man because of the fascist stance he took in the comics.
I am not saying this to pick on Iron Man, I've read the Avengers since the 80's, obviously I'm a fan. Plus I have the new Slott Iron Man series on my pull list. My point is that the movies made all the characters popular and mainstream again, including Iron Man.
So this claim that Cap needs other characters to sell is really bs. It's a shared universe, obviously we're going to see characters pop in and out of each other's story. Moreover, Cap's history has always been closely tied to SHIELD, even in the the comics. He's worked for them on and off since Kirby brought him back in the 1960's. In the comics Fury is an old war buddy of Cap's (and the leader of the Howling Commandos while Steve led the Invaders)! It was a natural fit, from a storytelling perspective, to put Widow in Winter Soldier since they both work for the same organization. Hawkeye was originally set to cameo too but they couldn't fit it in.
If anyone is ever in doubt over Steve's popularity and ability to sell based on his name alone, walk into Target and count the sheer number of Cap merch available. Seriously, walking around I see more Cap t-shirts than any other superhero.
The Russos originally intended to do Kirby's Madbomb story for Cap 3, they said this themselves to Entertainment Weekly. Markus and McFeely mentioned in their 2016 SDCC panel that they had started a different script for Cap 3 before switching to Civil War, they've also indicated that the beginning scene in Lagos, where Cap and team are preventing Crossbones from stealing a 'deadly virus' was brought over from that original script. Feige decided on Civil War because DC announced they were doing B vs. S, it had NOTHING to do with Cap not being able to sell a franchise on his own.
Plus it became a really convenient way to have the Avengers split up to segue into Infinity War.
I'd also like to point out that even during the Avengers lull in popularity, during the early 2000s, Steve has ALWAYS been popular with the troops. Marvel used to send Captain America comics overseas as a morale booster. They might actually still be doing that, don't know, I'll have to ask my Navy buddy. God, I hope they didn't send Secret Empire.