It’s not only utterly brutal on cash flow, it’s also extremely capital intensive. Which is why a few people with access to large amounts of capital (like Udvar-Hazy, and GE) made themselves incredibly wealthy by allowing airlines to convert those massive capex into much more manageable opex.

There is a saying is that “if it flies, floats, or f🤬ks, it’s a lot cheaper to rent it”. This applies to the airline industry in a big way.