Personally I don't see much use for passive devices in this day and age when there are so many chips available for very little cost.
Note that it is not only monetary cost that matters, but a processor would also use power. Adding a processor that allows networking and
a convenient user interface would quickly double the power usage in the case where mUPS is used to power a small MikroTik router!
(which is based on similar processors)
So that feature would halve the runtime of a given battery.
Of course there are processors that use a lot less power, but they usually do not offer networking. One could consider using a microcontroller
with USB interface (that presents itself as a USB serial device) and plug that into the USB port of the router, monitoring it via some simple
serial commands that are issued from RouterOS via scripting or some native code. But that would restrict the usage to routers that have
an USB port and nothing yet plugged into it.