Customer behavior is key here. There’s perceived risk with the dreaded “Multiple airlines” option when booking tickets, especially for international flights. No-one wants to find out the hard way that there weren’t agreements or reciprocal status or matching luggage allowances, etc. The code-share is shorthand for “we’re responsible”.
It's safe to assume that if you book on Delta's site then any routings they offer you will have interline agremeent.
Let's say you are traveling from Incheon to Atlanta on a flight with a Delta number. Would you go to the Delta desks to check-in? Maybe... is it a real Delta flight or a flight operated by Korean Air but marketed by Delta? I think getting rid of codesharing would make it much easier for travel novices.
There are so many agreements for crediting frequent flyer miles that while yes it can help reassure people, there is a million more routes that still credit miles without codeshares to the point where it's useless
