> you just have to provide USB-C option as a fallback.
Given you must have a USB-C port, the design would still be limited by the USB-C form factor and adding another port would even further increase the complexity, costs and weight of the design. This will still strongly limit the spread of new designs, and I'm pretty sure it's effectively equivalent to mandating just USB-C for smaller devices.
I see how this could be an issue for small devices, but this wouldn't stop a company from testing a new connector on bigger ones: laptops still have one or more USB-A ports, despite USB-C being common now. So having an additional connector other than USB-C isn't an issue.