Can a USB hub just be a simple bus?
It can do, yes. But it wouldn't be of any use as a USB hub. 

You could only connect one active device to it at a time, and even that would probably only work as a low-speed device because of all the trace stubs to the other ports.

If you plugged in a second device, it would either simply cause both devices to fail to enumerate, or if you are unlucky (and more likely), cause damage to one or both of the devices and possibly the host port.

You could use the other ports for providing power, but if that was the end goal there would be no point wiring the data lines to the other ports. In fact there would be no point connecting it up to the host port at all.



TL;DR

Don't do that...