The youngest 737s will still be around well into the 2030s
Not if they exercise all of their options before that timeframe.
They won't, they'll likely do them gradually. Qantas retires aircraft at around 20 years, due to the way depreciation works in Australia. If they could depreciate their aircraft faster (like other airlines are permitted to do under local laws), they would retire them faster.
Qantas can write them off faster under Australian tax law - if their carrying value is above current replacement cost. Can’t imaging 738s will hold their value.
They did it with some of the 747s, to take advantage of the big operational loss around 2015. Nobody notices a big extra write down when the headline is a big operational loss.
So at any time, if the value that their 738s are being carried at (cost less depreciation to date), is greater than what they could replace them with of similar age etc ie the markets drops then they can write off the difference, or sell them, with a loss recognised. They are required to do so or the auditor could qualify the audit opinion on the financial statements.
The bottom line is Mr Joyce won’t be hanging on to his old 738s unnecessarily when he has nice new flexible A320 family aircraft to carry customers, including those avoiding VA’s and Bonza’s MAX 737s.
Unfortunately for Boeing, another example of them getting exactly what they deserve (re 737 Max, KC-46A and 787).