Napes
When I lived in Annandale, I used to ride the 29 express buses regularly. I rode the 16’s several times on weekends (when the express buses do not run) mostly to move by bike to more bikeable places. I think I took the 16 on a weekday once or twice.
Certainly on weekends most of the ridership on the 16 from Annandale got on at Culmore or closer in – and note, most 16 buses did not originate in Annandale, so I was not seeing the folks getting on the buses that originated closer in) And some of the people who rode the 16 from Annandale got off well before the Pentagon. Assuming something similar held on weekdays (though of course there would be more people going all the way to the Pentagon then) I doubt that establishing local fares on the express buses would do a lot to reduce demand for the 16 buses.
Its certainly worth exploring (though I guess the subsidy would have to come from Fairfax, not Arlington, and there would be problems with inconsistency with other express buses) now that the streetcar is dead. But I do think the answer to the bus congestion on Col Pike is going to have to happen on Col Pike. The best approach now is to speed the implementation of the enhanced bus option.