The definition of mainline is an aircraft operated under the Delta Air Lines operating certificate.

I'm not sure what operating certificate would be used if a Delta pilot flew a 100 seat airplane owned and branded as "ComAir". I guess it would still be ComAir. Of course, at some point you have to stop calling them regional flights and start calling them partner flights or codeshare flights (or just "not mainline").

I've always wondered why someone didn't start an airline (or convert an existing one) that is just a piece of paper and contracts out everything. It would be like reinsurance for airlines.

If a union goes bananas (the insurance equivalent to a tornado hitting a mobile home park with a stack of policies from the same insurer), you chuck them and get yourself a new carrier.