I haven't used spreadsheet software in decades, but I have helped some convert to Windows to Linux. Some of them did use Excel, and therefore had to learn to use LibreOffice Calc, and while they had some expected difficulties during the initial learning curve, they did say a few months later to me that they were eventually satisfied with the software.

Nevertheless, I'm sure much like the GIMP/Photoshop comparison, Excel simply has features that Calc doesn't.

I am mildly curious. Could you give an example of a feature that its likely many businesses and/or individuals use in Excel that simply doesn't exist in, or is too difficult to implement in Calc?

I would imagine the vast majority of Excel written everywhere uses very basic features.

Not necessarily. It's often less Calc's capability that is at issue, and moreso its compatibility with imported sheets. Calc tends to have every feature I need when I make a spreadsheet.