They can do things like rely less on leasing airplanes (more full ownership) and carrying less debt ie more conservative growth.

Having that much excess cash with no leverage only makes you a prime target for some mr. Icahn to take you over when hard times hit you.
And even with 18 billion in cash right now, the airline would have 0 dollars in 6 months, and would be in even more serious trouble.

There isn't any kind of buffer an airline can have to prepare for something like what we are experiencing right now.

Paying down debt would actually reduce cash in the bank, but it also means you bleed less cash every month. So if/when covid 2.0 hits you don’t have to worry about making so many lessor/debt payments every month.

A paid off aircraft that is parked doesn’t make you any money, but it bleeds little cash. Basic insurance and parking and minimum upkeep/maintenance. A parked leased aircraft is going to cost you tens of thousands if not hundreds of thousands every single month.