Airlines with large aircraft operating under 14 CFR 121 operate in three
arenas. Domestic, Flag, and Supplemental. Supplemental is the FAR 121
equivalent of charter. Operators of smaller aircraft operating under 14 CFR
135 tend to be either scheduled or on demand. On demand is charter.

All the other points in this thread are valid. Specialization tends to be
more rewarding than being generalized. The frenzy of the 70s and 80s to have
companies do everything for everyone didn't achieve much general benefit to
those companies nor the consumers, and those companies have spent the last
10 years divesting themselves of those acquisitions.

Simon