https://www.aerotime.aero/articles/southwest-pilot-union-concerns?

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As the carrier has almost entirely remained a Boeing 737 operator throughout its 57-year existence (it operated a small number of 727s in the early 1980s), it may be that the Southwest board could resort to the acquisition of another US airline to solve its fleet issues in the short to medium term, according to Murray. 

Murray adds that Southwest’s over-reliance on the Boeing 737 range brings “a level of risk” to the company, adding that recent events involving the type have resulted in many in the industry now questioning if the 737 MAX 7 will ever be certified.