You wouldn't catch BA for example operating BA123 MAD-LHR and then a couple of hours later BA123 LHR-JFK on a totally different aircraft... there's no reason to do it...
Good grief, learn a little more before you pontificate. In the US they schedule some of the more poular connections on a single flight.
Hence GLA-EWR EWR-LAX, both on CO017 calling as COA17 but with an aircraft change. Same flight numbers / callsigns with an aircraft change are common. The whole BA European regional operation was built like that.
BA1234 GLA-BHX/MAN
change aircraft
BA1234 BHX/MAN-Europe