To me, the question sort of misses why (I believe) people use Excel instead of something else. I think people use Excel for all the things that are missing from other spreadsheets. Google gives away its spreadsheet and markets it to everyone with a Gmail account and makes it easy to use and people still pay for Excel because the cost of the missing features outweighs the price of the paid software.

Sure there are people who will express dissatisfaction with Excel. And many many use cases for which Google's product is good enough. But it is all the edge cases that Excel covers that makes it viable to standardize upon across a user base where some users have a significant level of expertise and that expertise is diverse.

To put it another way, free and available anywhere and easy collaboration are all great features. But they are not enough. Killer features will only become killer once a spreadsheet does pretty much everything Excel does (and in pretty much the way Excel does it so that muscle memory works).