Note to self: an Oracle trace file must still be named its original name if you want TKPROF to accept it for processing.
I received two Oracle trace files, after both had been renamed to be more meaningful in the current troubleshooting context. I then churned butter for 30 minutes, trying to understand why TKPROF would not do anything except say "could not open trace file". Eventually, I thought "maybe it requires the original tracefile name?". YES!