It was not so much about udev hwdb being a must. It was just about presenting the reasons it's being used for by libinput.
Also, libinput is not just about "slightly more tunable touchpad settings". It's basically the successor of evdev/synaptics/wacom and so on. As per the presentations mentioned above, those older drivers are basically unmaintainable, because they have such a messy code, which is why they are no longer being actively developed. libinput is the successor, i.e. it gets all the improvements, fixes and features, whereas the older ones do not. The presentations also mention real-world advantages that libinput has over the older drivers (such as for example being able to have a context between various classes of input devices, which is not possible with the older drivers, since they are separated from each other).
But you might be right that udev hwdb is not so essential after all, if all it really does is providing a database for devices which allow libinput to automatically adjust it's settings for specific devices.
When the same can be achieved by manually adjusting the libinput settings (via xinput), then it might be worth to go without udev hwdb and save those 6 megabytes.
That being said: It would be nice if you could add xf86-input-libinput to the repo (without udev hwdb) for the reasons mentioned above.