Remakes should be done if improvements can be made upon, or if something new can be brought to the table. I do believe there are some films that are "untouchable," not in the sense that they shouldn't be remade, simply that the original film is perfect as it stands and remaking it is needless. Is Oldboy deserving of this? I don't think so.
The original is incredibly overrated. Chan Wook Park is a horrid screenwriter, and that's where the film ultimately fails. The film has amazing cinematography and direction, there's no questioning that, but the rest of the film isn't so flawless. First of all, I'm bored to death of Korean scores, which basically amount to classical music 99.9% of the time. Second of all, the acting while damned good for Min-sik Choi (except the end), was overly melodramatic and overstated in a lot of areas. It was quite cartoonish in this respect, which is understandable considering its an adaptation of a graphic novel, but not in line with the overly serious tone the film ultimately takes. The villain in particular gave one of the most laughably cheesy and tone contrasting performances that I've ever seen. The narrative was disjointed as all hell and the actual ending, and not the twist, is honestly one of the dumber things to grace a 3rd act.
Basically, the film can be improved upon, which is the only justified reason for a remake IMO. There's room for an Oldboy film that maintains the excellent visual flair of the original, with a lean narrative, strong performances across the board (and an actual villain this time around), decent soundtrack and an ending that isn't an insult to the audience's intelligence. And it's bringing in elements from the source material (manga) that was not present within the original adaptation.
It's a warranted remake IMO.