> I was once told that the tab key can be represented in different ways on different systems, and that's why spaces are safer because they're always represented the same.

The main counter argument is that users have different preferences for the amount of indentation, so giving them control over that, just as they (nowadays) have control over the font used and window width, is a good idea.

The tongue-in-cheek counter argument is that fixed-width spaces are preferable over ‘normal’ spaces. They also give you more control over indentation, allowing, for example, mixing usage of THREE-PER-EM SPACE (https://unicode-explorer.com/c/2004) for indentation with FIGURE SPACE (https://unicode-explorer.com/c/2007) for right-aligning numbers.