Jim,
You are 100% correct. Nobody would even attempt to put a date on a motif.
As you say most of them go back to the earliest records and as far as we know beyond this.

However, there is some mileage for researchers in identifying the earliest extant appearance in ballad form, even if that earliest appearance is in another culture or state.

Dating the earliest extant versions is not easy by any means but I find it a useful exercise and other researchers appear to be interested in the results.

Takes all sorts I suppose.