Rafa not winning WTF is not the main cause of the event's lost significance. But WTF no longer has the prominence it once did.
En masse the players just don't feel the same as they used to about the year end event, and it is struggling for relevance. The exhibition season is lucrative. Most players don't want to be stuck in London when they can be playing a more relaxed exhibition event with a format of their choosing somewhere more exotic for more money.
The off season is so short and Australian Open preparation is so important. Again, why be stuck in London when everyone else is getting a step ahead on their recovery / training block prep work or resting injuries.
The o2 iteration of the WTF event was a cash cow for the ATP, but they overstayed. The o2 "style" of molasses slow indoor hardcourts, cold neon blue lighting and players exhausted from a hectic season has made the event pretty boring to watch.
The ATP putting 1500 ranking points on this event is a form of life support. No one would touch it otherwise.
As much as I love the idea of seeing the WTF come back to life in Turin, I partly wonder if the busy calendar, short off-season and Australian Open's raised significance so early in the year have made a season end style event feel slightly redundant.