Around 1970, ABC was host to a trio of new soaps ("All My Children," "A World Apart," "The Best Of Everything") and existing ones like "One Life To Live" and "General Hospital." So by the time "Dark Shadows" came on at 4pm, die-hard ABC daytime fans had already watched 5 other serials (and a few game shows thrown in the mix) - so I wonder if this excess contributed to the eventual demise of DS.
In the mid 70's, "The Edge Of Night" moved to ABC and was put in the late afternoon time slot like DS, following 4 other soaps (3 of them an hour long), and it too gradually lost viewership and was cancelled.
ABC had good "sneak previews" between shows by '77, offering an exciting snipet of the following soap's story for that day, so viewers would stay tuned. Does anyone remember if they did that for DS? All I recall is during the end-credits for "One Life To Live," the announcer would say "Now stay tuned for DARK SHADOWS, next on most of these ABC stations."