The problem with Sam Wilson as a character is I feel like he has peaked in terms of popularity. People like the character. They might even feel he is a good Captain America. But it's hard to look at him at this point as being something more than a support character.
TFATWS was designed to have Sam remake himself into a new hero, thus bringing an end to his "support character" identity. He was no longer a sidekick, but completely transformed himself into
the American superhero icon. With Rogers long dead (and not competing to be a second, active Cap), Sam was reborn as Captain America, who should be the leading face of the MCU going forward, for all that costumed half and most importantly--the struggle he's faced (and might continue to face) in order to become this new identity in a world where he will be rejected or not taken at face value (when they did so with Rogers).
Plus Chris Evans was so good he is basically going to be the template for a very long time when it comes to Captain America. Some of these character have become so popular that finding replacements for them will be hard.
The flaw in your statement is that Mackie is not stepping into the role of Steve Rogers (in the way various actors have been cast as Bond). His Sam Wilson has taken on the Cap role, remaking him into a new character with his own motivations, which one would
hope the audience can understand.
I am not saying Sam Wilson will be a bad Captain America. More like I am saying it's hard to see him being as iconic in the role as Evans version of Captain America. Which means that even though he still can be used very well in the MCU we can't really expect him to sort of be part of a big 3 or big two are are basically the faces of the MCU. To uses a sports metaphor, he is still a role player as opposed to being one of the big stars.
This is the long-worn argument similar to those made by Right Wing zealots in social media--that
for some reason, Sam can wear the Cap costume, but there's just something about him which forever relegates him to "B-list" player who should never be seen as one of the "Big Three".
Of course, this ignores the fact that when the MCU began, each character--Captain America, Iron Man and Thor--were
not (in film recognition terms) established "big" / lead characters, but through franchise runner choice, they were allowed to
grow into that position, yet some seem to operate from a few things, such as selective memory about
where the so-called "Big Three" were at the dawn of the MCU, and how Sam Wilson is more recognized at the point of his taking on the Cap role than his predecessors were.
In the end, it does nt matter, but Disney will be setting their own firestorm if the next Captain America is not the leader he should be, but is shoved in any sort of supportive role to characters who are in no way as recognized, or important to the legacy of one of the flagship Marvel characters.