I think they are bringing back Captain America no matter what. The way that they ended Evans run on film allows for two options (if not more):
Option 1: You get someone of similar type to play Rogers as an old man as they did in the comics. Falcon has the Shield but Rogers acts as a battlefield commander. When it came to the Red Skull reappearing, Ross Marquand demonstrated that with a decent performance and makeup you can take over a role as if it was yours to begin with. I am not suggesting Marquand at all, but Marvel can find someone.
Option 2: Captain America is deaged and appears as Justin Hartley or any other actor. Marvel unapologetically will cast a new actor in a roll and not only make it work but have us thanking them for it. Hulk and James Rhodes immediately come to mind. While swapping Evans for someone else would be a much more noticeable course of action, I believe it can be done and I believe it will eventually be done. I have no problem and would actually like to see Mackie carry the shield though for at least a few years and a few movies past Falcon and Winter soldier. Why not forever though? Captain America has always been about a man out of time. This was represented in Winter Soldier with his list of things to catch up on and even in Endgame when he decided to live his life... back in time. The story of Captain America, at least on screen, is taking the stereotypical elements of the Greatest Generation of WWII and fighting to make his moral code and sense of honor exist in a world that is not only devoid of it but in some aspects rejects it.
I want to see how they apply this to Falcon. If they can come up with a similar struggle then keep Falcon forever. If not then they need to eventually. bring Steve as a WWII soldier back.
Some characters work regardless of who is wearing the costume. For example, Frank Castle could be anyone of any race or gender so long as their family is murdered and they are ex-military. That is who their character boils down to. "one bad day"
I think the hardest thing to ever recast is RDJ. That is a character who is now defined by the actor who has played him.
Now I am rambling.