Liqudated:
Braniff
Braniff II
Pan Am
Eastern
Midway
Midway II
Via mergers/buyouts:
Western
TWA
Piedmont
PSA
Hughes Airwest
I am sure many others that aren't at my finger tips.
Dozens, if not over 100, who just weren't big enough to count as a major.

Nothing is a "sure,secure thing"
Not even UAL or SWA.