Similarly, if people north of Wealdstone want to go to Euston, perhaps
they would be better served by a train which didn't stop at all stations
south thereof?

Perhaps worth noting that it's possible for them to change at Harrow &
Wealdstone for a Silverlink County non-stopper to Euston (although
currently there aren't always reasonable such connections in the
timetable). Not much use though I suppose if the shared running with
the Bakerloo makes all the trains late thus missing the connection.