Although SW:GE may impact some guests' plans to come next year instead of this year, I'm skeptical that it's driving the numbers down now. We've heard that a slow start to this summer might have resulted from TSL's opening and pushing people to go after June 30 (which may not have been the case after all). We're hearing now that SW:GE is causing lower numbers for the 2nd half of 2018. If the numbers are below expectation towards the end of next year or early 2020, I'm sure we'll hear that it's because people are waiting for the 50th Anniversary.
It'd be just as easy to say that many people were waiting for Pandora to open this past year, that its debut lead to higher than average attendance, and that crowds now are returning to normal (but that's looking backwards and not as exciting as the anticipation of guests waiting for the new thing).
There's always something new around the corner, but not many people shift vacation schedules around (by a year or more no less) to accommodate those debuts. Not to say some people don't - just not the vast majority of guests (many of whom may have only the faintest idea of SW:GE - if they're really aware of it at all). There are any number of reasons for why people aren't booking - and not all of those are as charitable to Disney (and many of its recent policies and price increases) as anticipation of a new land.
