Although I definitely agree that Word and friends are good products.
One of the biggest reasons the company I was working with at the time made the switch was sheet size, we came up against the 65536 rows limit at least once. Formula calculation speed was another big one. In many ways the ribbon does feel like a step forward, perhaps the only thing 2003 still does better is keyboard shortcuts, but luckily you can still use Excel 2003 keyboard shortcuts in newer versions if you can remember them (Alt,d,f,f and Alt,d,f,s are probably the ones I use most frequently).
At which point you have a database and would be better served with a different product designed to work with large data sets or moving data to external ODBC sources that Excel can query.
Because the license cost savings are so small compared to the payroll costs of the people using the software that even a small hit in efficiency from existing staff (and new staff as rotation occurs) being less familiar will outweigh it quite easily.
Heck, its also why lots of firms give a pass to major version upgrades even when their license would allow the major version to be rolled out at no additional license cost until the version they are on is old enough that it is causing a drag because any new staff are having trouble converting back to the old version.