It seems that Delta Air Lines will be the only US global FSC to operate a 100-seat aircraft on the mainline for the foreseeable future. United has no immediate plans to add small narrowbodies to its mainline operations, whereas American is shedding its fleet of 20 99-seat Embraer 190s in 2019.
Although United has evaluated 100-seaters in the not too distant past, the airline does not seem poised to add an aircraft in that seat range to its mainline operations, despite perpetual fleet evaluations.
Delta seems to have found a formula to operate small narrowbodies on the mainline profitably. The 717s replaced more uneconomical 50-seaters and arguably deliver more revenue, given their three class configuration. The airline has ordered next generation Airbus A200-100s (formerly the Bombardier CS100) to replace some of the 717 narrowbodies…Fuente: https://centreforaviation.com/analysis/reports/delta-air-lines-the-only-us-big-3-airline-taking-100-seat-aircraft-431837