In theatre, actors grow old and eventually die. Every new generation of actors can find its own greatest Hamlet. But with the advent of the cinema, it became possible to preserve the best ever portrayal of a given character. Stephen Fry is the one true Jeeves, and Hugh Laurie the only Wooster (though it’d be fascinating to see them swap roles as I’ve heard was originally intended). Leonard Rossiter is Reggie Perrin (Sorry Martin, you’re talented, but even you couldn’t pull it off.) And Jeremy Brett is the definitive, nonpareil, Sherlock Holmes. Robert Downey Jr as Holmes is pointless.
Edited to add: And remaking Troy Kennedy Martin is usually a mistake. The Italian Job wasn’t his greatest work: it succeeded by dint of bizarre casting (Noël Coward and Benny Hill?) and music. Only a fool would remake that. Edge of Darkness, on the other hand, arguably was his greatest work, perfectly cast and directed (though some of the Clapton mood music was loud enough to jar). Only the most hubristic fool would attempt to remake that.