I think Ramsay simply has to die at this point for the plot to move forward.
Lets break the main characters into three groups - Good, bad and grey (ie - could either end up being a hero or a villain at this point).
Good:
-Jon Snow
-Sansa Stark
-Brann Stark
-Brienne of Tarth
-Arya Stark
-Samwell Tarly
-Davos Seaworth
Grey:
-Tyrion Lannister
-Daenerys Targaryen
-Theon Greyjoy
-Yara Greyjoy
-Jaime Lannister
-Margaery Tyrell
-Tommen Baratheon
-Sandor Clegane
-Bronn
-Melisandre
-Jorah Mormont
Bad:
-Cersei Lannister
-Ramsay Bolton
-The High Sparrow
-The Night King
-Euron Greyjoy
-Littlefinger
Ok, so the show is clearly moving towards two things - Daenerys crossing The Narrow Sea and the White Walkers invading from the north.
If we look, all of the 'good' characters are more or less on the same side, and have either come together or are in the process of trying to do so (both Arya and Brann are trying to get back to Jon/Winterfell as of the most recent episode).
However, almost ALL of the bad/grey characters still have different motivations. In this show, that is not necessarily a bad thing however with next season only seven episodes long, and the following season rumored to be not only only six episodes long but to potentially be the final season, that leaves with only fifteen hours of television left.
How much more time can they devote to Ramsay running around flaying people in the north if all of those other much more significant plot lines have to come to a head over the final 15 hours? AND given he's gained the allegiance of several northern houses, if he is at large then Winterfell can't be the fortress we know it will have to be against the White Walkers?
TL;DR - They have to kill Ramsay because they're running out of screen time and have to focus on other things.
And, oh, I think Rickon is proper f**ked this week!