Note that single disks are almost for free now. 3TB or 4TB disks are very affordable. It's only an issue if someone invests money in a hardware box with room for a lot of disks on pure speculation that any real money gets locked up on pure speculation.
This is about a user who have a single 1TB disk getting full. So not about buying an 8-disk NAS preparing for 8*4 = 32TB of spinning platters just in case, but a question of considering a 3TB or 4TB disk instead of just a 2TB disk. If having extra space for the main storage and any backup disks, it's possible to store multiple edits or maybe an extra full copy of a laptop or similar without worrying that the everyday tasks suddenly are hampered by the lack of space. In the end, most people do see an exponential growth in storage needs, even if the coefficient might be different and the needs at this very moment might start from a lower value.
Having extra disk space is like having extra batteries or extra memory cards - it's no fun to run out at the wrong time.