I am not sure Alfaro will ever be great, but I am pretty sure his defense will be above average. He is already good at framing and throwing. The blocking should improve over time with some work.
As for offense, he may always chase too much. He did make some tiny strides in his discipline in the second half and his OPS went from 700 to 788. An Alfaro with good defense and a 750 OPS seems like a 2-3 WAR catcher to me.
There is a good chance that he does this. Which is why I don't want Ramos who might not be better than Alfaro. Realmuto is clearly better. I just see less need to improve at catcher at a high cost when there are other ways to improve the team while keeping our young prospects.
We are trying to win now AND in the future. Keeping Alfaro does not really hurt the goal of competing now, so why not preserve some future too. Trading for Realmuto as a reaction to not getting Harper or Machado would be a step backwards IMO. Trading the future for a move that is not likely enough to win today.