As a passenger, if my ticket says Delta, the ticket counter says Delta and the plane paint says Delta, it is a DELTA flight.
That's a true perception. Delta is responsible for putting its name on the product it sells. But there is a difference.
A Delta mainline airplane will have an average of about 45 years of experience in the cockpit. So will a Delta codeshare with another 'legacy' carrier. However, a Delta-branded flight utilizing regional, commuter, and feeder carriers operated by non-legacy carriers might have an average of 10-15 years experience in the cockpit, or often much less.
I agree that one of the things that sets a premium on legacy carrier tickets is the quality of service, which to me, includes the expectation of a highly experienced flight crew at the front end. The only way the customer can tell, though, is by knowing the airplanes the legacy carrier operates, and buying tickets that specify that equipment. Even then, subsitutes can occur.
I don't disparrage the crews or carriers of the regionals; we all have to build our time, and all pyramids narrow at the top. But I would like to see more emphasis at the point of purchase on who exactly will be operating the aircraft providing the service for sale. There is a difference.