I think they cling to 1914 because they have turned it into a legit prophetic moment in their history. The story they tell is of Russell discovering that 1914 would be the beginning of the last days and, lo and behold, World War 1 begins and a narrative is born. That narrative is easy to package and distribute: war begins, the world goes into decline, the only possible salvation is the kingdom of God with Jesus at the helm. Without the war, it's really just another end-times prophecy that does not convince anyone.
Even now, when it's pretty easy to find out that Russell's predictions were many, that none of them amounted to anything, and that only by radically changing one of them did the WTS create the 1914 'prophecy', they do not let it go. To do so undermines the strong narrative that they used successfully for so long. The farther we get from 1914, the tougher it gets to prop it up, but I think they're too far along to try and salvage it now.
I'm curious what they will do. I still think they will try to ignore it and minimize it as quietly as they can, hoping to cling to the credibility it once gave them but never actually having to face up to it.