From the article:
"Conceptual directors, who use their films to express their ideas or emotions, mature early; thus such great conceptual innovators as D. W. Griffith, Sergei Eisenstein, and Orson Welles made their major contributions early in their careers, and declined thereafter.
"In contrast experimental directors, whose films present convincing characters in realistic circumstances, improve their techniques with experience, so that such great experimental innovators as John Ford, Alfred Hitchcock, and Akira Kurosawa made their greatest films late in their lives."Plumer goes on to basically agree with the premise, referencing an earlier paper by the same authors regarding paintings, and admitting at the end, "Not knowing much about movies, I have no idea if this theory is even remotely plausible." Full disclosure here, I am not in any sense a hardened professional here; I am merely am amateur fan of the filmic arts, but I suspect that the paper has little to no validity.
The abstract quoted above calls Griffith, Eisenstein, and Welles "conceptual artists", defined from the painting paper as those who define the work primarily during the planning stage, whereas experimental artists work with their chosen medium and work towards further perfection by examining the canvas in-progress. I would agree that the greatness of those three directors lies in their conceptual abilities, for they innovated many of the techniques that have been used in cinema ever since (or at least perfected their use from earlier experimental shorts), but it's also true that the artists, Welles in particular, did plenty of fine and innovative work later in life. Welles is best known as a filmmaker by his Citizen Kane (1941) and, to a lesser extent, The Magnificent Ambersons, made one year later, but a list of his greatest films must also include Touch of Evil in 1958. A fairly recent documentary, Orson Welles: The One-Man Band, shows much of the later work of Welles, after the studio system had written him off, and the result is a man who, in short home-movies and the like, continued to innovate for his entire life. Certainly his acting skills did not deteriorate, and the brief glimpses of his final picture (The Other Side of the Wind), still uncompleted and unreleased, show a filmmaker still being boldly experimental, still pushing the envelope as much as ever, and using few if any of the cinematic techniques he perfected in Kane.
So far as the other side of the equation, I'm not sure that the "experimental" filmmakers listed in the paper make any more sense. Ford was a standby of the old studio system for decades, making film after film -- his greatest success is arguably The Searchers from 1956, but equally solid claims can be made for Stagecoach from 1939 or The Grapes of Wrath from 1940. Hitchcock was active in animation for years before getting the chance to direct his first features: he brought the standard animation technique of storyboarding over to live-action films, and is now a standard part of any filmmaker's process -- his greatest films were scattered throughout his life, with his greatest peak near the middle of his long career with Vertigo, Psycho, and others. Both men had their greatest successes at the end of their lives not because of their process, but because only towards the middle of their careers, when their box-office potential had been proven and the old studio system started to crumble, were they set free to do the work they had always been capable of doing.
Kurosawa is another odd one to add to that list. Arguing that his greatest works were at the end of his life is futile: is Ran (1985) superior to Rashomon (1950) or Ikiru (1952)? If the measure is innovation, I'd argue that Kurosawa would belong in the other category, of "conceptual" filmmakers, whose works speak of enormous ideas that are merely enacted on-screen -- Rashomon was so unique for its time that it has been imitated hundreds of times, and the word itself acts as an adjective universally recognized. The relative recognition he received late in his career was due to political problems in his native Japan, not a lack of innovation or a problem with his methodology.
In the end, I believe that this way of viewing innovation in film is more-or-less useless, except to the degree that it's obvious. Every student of cinema quickly learns that many of the medium's greatest innovators peaked early, but circumstances surrounding the lack of innovation in later years revolves as much around biography and politics as artistic merit. Scorsese's great films dot his resume; from Taxi Driver to Goodfellas to The Aviator, he continues to grow and adapt to new technologies and methods. Altman invented new technologies and methodologies for M.A.S.H. and Nashville, but he's still using them to great effect decades later in Short Cuts and Gosford Park. Steven Spielberg had great innovative work in the early days of his career, with Jaws and Close Encounters of the Third Kind, but who would argue that Schindler's List and Saving Private Ryan are not just as high-quality and innovative in their own way?
Innovators in film generally continue to innovate during their entire lives, unless personal circumstances deny them entry into their respective fields, those artists who start strong tend to continue doing so, and those who start in a lackluster way tend to work in the same area. The only counterexample I can think of is Woody Allen, who started off doing silly comedies like Bananas before moving on to more complex and sophisticated fare like Manhattan, but even then that seems to be a matter of personal choice than of lack of innovative method. I'll continue to try to think of a counterexample, but for now I can't really think of any.