American Airlines CEO Doug Parker said the company was “frustrated” with Boeing on Thursday, referring to costs incurred by grounding its Boeing 737 Max jets following fatal crashes in 2018 and 2019.

“We are not happy,” echoed Southwest Air CEO and Chairman Gary Kelly on CNBC.

Kelly noted Southwest’s board asked him to explore “diversifying from having a sole supplier.” Southwest only flies Boeing 737 jets, including older models that came before the 737 Max.

But adding a new model would be costly. For instance, pilots that fly different aircraft would no longer be interchangeable.

“You need to maintain separate pools of pilots,” Samuel Engel, airline economics and strategy analyst at ICF, said. “It’s not so much that it costs you more to train a pilot in one aircraft type or another. You can no longer mix and match those pilots when you’re building rosters.”

New fleets would also come with new maintenance costs.

“There’s different types of certification required to maintain the different types of aircraft,” Kevin Michaels, managing director at AeroDynamic Advisory consultancy said. “You now require different types of training if you maintain two different aircraft models.”

Changes would not be quick. The waitlist for the Airbus A320, which competes directly with the 737, is more than seven years, Michaels said. Given all that, Michaels and Engel assume airline executives’ complaints about Boeing are negotiating ploys to extract higher compensation fees from Boeing for grounded aircraft and cheaper prices for future purchases.

On the other hand, it’s also expensive to stay with Boeing. Right now, it’s unclear when the 737 Max will return to the skies.

“Are passengers going to be willing to fly the Max?” Christine Negroni, aviation writer and author of “The Crash Detectives” said. “Every time there’s another explosive story about the Max, there are people who say, ‘You know, I don’t know if I want to fly it even when it’s released to fly.'”