Purely speculating, but I always assumed that the rationale for code share flight numbers must have been something technical, e.g. the airline code indicating which airline’s database a given record is located (ticket, bag tracking code etc).
It’s probably not strictly necessary anymore today (as you say, I’m pretty sure I’ve had a ticket across two carries without a codeshare number), but maybe it was at some point?
It’s probably not strictly necessary anymore today (as you say, I’m pretty sure I’ve had a ticket across two carries without a codeshare number), but maybe it was at some point?