Falcon as Cap isn't the issue. Sam Wilson is and always has been a great character. It's wrong time, wrong place.
They successfully transitioned Bucky into the Cap role and then forcibly shoe-horned Steve Rogers back into the role just because the movie was coming out, a sell out move IMO.
They could have left Bucky as Cap, left Steve as in the Commander role then did the same story with the Iron Nail aging him putting us in the same place instead of a retread plot only a few years after the first time they did it. Instead Falcon is in the role which is getting a lukewarm reception at best and Bucky is left in limbo sitting on a wall in space.