Ribbon-era Excel had massive take up for a number of reasons, but there was certainly reluctance to switch (from what I saw at the time).

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).

> we came up against the 65536 rows limit

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.

This is true but empirically, nobody cares: they're gonna do it in a spreadsheet. So spreadsheet software has to allow for this massive use case, and arguing they're doing it wrong gets no traction.

That only works if you have the IT infrastructure to support it. In the company I was referring to before, the IT team was effectively one guy. Plus, there was a considerable Excel expertise in the team I was in at the time, aside from calculation time Excel was working very well for what was needed.