Film critics... tend to favor the paradigm of filmmakers hitting their artistic peak in early or mid-career, and thereafter laboring in vain to capture the flair of their youth. It's a natural attitude for the reviewer on the beat, deluged with mediocre contemporary product and doomed to wistful fantasy about golden ages. Implicit in the myth of decline is a vision of movies as mechanisms: uncanny objects that somehow "work" or not because of the confluence of ineffable forces. And the late films of many directors seemed often not to "work."
Auteur critics set out to substitute for the mechanism model an artist-oriented aesthetic that was borrowed from older art forms that had clearer claims to being the expression of individuals. Instead of working or not working, movies partook of the creative personality of their directors, and a valuable personality almost inevitably imparted value to individual works. Part of the auteurist agenda was to find a new mythology for the late periods of directors: where most critics saw the pitiable flailings of age, auteurists often saw the mature work of artists who had become too serene and wise for their public to keep up with. The paradigm of the early peak is still pervasive, and if auteurists courted ridicule by playing the "late period" card too freely, they often enough succeeded in installing late works in the canon.
I don't think you need to fully embrace auteurism to appreciate that the "work"/"doesn't work" model of looking at movies (or arts & culture stuff in general) is extremely limiting.