Steve's my favorite character in the MCU, but I don't really understand the heated arguments for/against the idea of passing the mantle. I don't have a problem if the title dies/retires with Steve, but I don't have a problem if for some compelling reason, Bucky took it up as a way to honor his friend. Any good writer could make it work, and it actually opens up story and character possibilities for Bucky.
I also think the stuff about how Bucky can't handle a franchise/outlive Steve because he killed Tony's parents is way, way, way overblown. First of all, he was not in control of his own actions and by A4 he'll have 3 movies as a protagonist.
Loki is a beloved MCU sex symbol and comic relief, and he's a MASS MURDERER. Bucky is in a way better situation, character-wise.
A) Steve's arc isn't done. As a Cap fan I would feel cheated if they ended it now. For one, the entire point of the Nomad storyline and the entire theme of the character - a huge recurring theme that has been brought up again and again in the comics: Englehart, Stern, Gruenwald and Waid have ALL addressed it and they are arguably the greatest Cap writers of all-time after Jack Kirby - is that Steve does not wear a suit with the American flag on it because he supports America as it is or the government, he wears it because he supports the
ideals of America, supports what America
should be, the dream. The entire point of the Nomad storyline and then The Captain/US Agent storyline and then later Civil War was Steve coming to this epiphany, which he hasn't done yet in the MCU for all that they've made him Nomad.
Secondly they introduced the Accords in the MCU. In the comics, after the Secret Invasion/Dark Reign/Siege storyline Steve is the one to defeat Norman Osborn. He is given the title Commander Rogers by the president, and becomes America's Top Cop (the position Osborn held before Steve), and the first thing Steve does in this new position, the very first, is get pardons for all who was on his side during Civil War and overturned the SHRA. Which is a second thread, in addition to Nomad, that needs to be tied up in the films. And in my opinion these threads are Steve's to tie up.
B) Legacy characters have a long history of not working. I've given numerous examples. There's also the fact that Steve is in the top three for selling merch while no one is buying any Winter Soldier product. I go to SDCC every year. And Funko does this thing for SDCC where they release special edition Funko Pops specifically for the con. Last year, for 2017, they had a Bucky!Cap Funko... it was the only special edition Funko that didn't sell out.
Moreover, Steve is one of the few characters Disney uses to appeal to their most lucrative demographic... children. Before Black Panther came out and for the longest time, Disneyland here in Southern Cali only had three Marvel costumed characters walking around the park on a consistent basis: Cap, Thor and Spider-Man. Now they've added Panther as well (I have an annual pass as a S. Cali resident). The reason? Because these characters are the most
family appropriate. Loki was never intended to appeal to the younger demographic. Cap is. There is no way they are going to put a character who was shown onscreen killing the Starks into a mantle that Disney uses to appeal to their youngest demographic (and the entire reason they bought Marvel to begin with, btw. They figured they had the little girls in the bag with their princess line, they wanted the little boy demographic too. Iger has said this himself).
C) And this one is just my personal opinion. I don't want Steve replaced. To me he is not replaceable. He's one of a kind. Unique. I resent Brubaker and the entire Bucky!Cap storyline. That and I find Bucky as Cap to be about as interesting as watching paint dry. In comics, while he was Cap he spent the entire time whining, being emo, and also being a general nuisance as Sam and Natasha had to consistently save his butt from scrapes. Now, again, this is just my personal opinion, but seriously, I don't want to watch that on screen. I have zero interest in that. Your mileage, of course, may vary.