So if the Yeti broke in mid-2007 then it never had a full calendar year from January to December where it was fully working. One of the arguments that has been made is that there hasn't been a drop off in attendance or ridership since the Yeti has been broken.
I'd say that it's conceivable that the attendance could be higher with a fully functional Yeti:
2005: 8,200,000
2006: 8,910,000: Expedition Everest Opened April 7th
2007: 9,490,000: Yeti broke in the Summer
2008: 9,540,000
2009: 9,590,000
2010: 9,686,000
2011: 9,783,000