Well, the difficulty with excessive debt is that as the credit rating goes down, the debt service tends to go up. Nothing wrong with taking on debt for an asset that can be used to make money in the business. It’s like buying a house to rent, as long as the rent payment a little more than covers the mortgage, it’s OK. But the problem with having debt in this environment is that - through no fault of the borrower - the ‘rent’ just isn’t there.

Worse yet, you got good terms from the lender because you collateralized those bonds with shiny new airplanes at a time when the waiting list for new aircraft was long and the market for used aircraft was similarly high. But it will be five years (or ten years) later pretty soon and - again through no fault of the borrower - COVID has kept those aircraft from being gainfully employed, increased the debt, and driven down the liquidity of the borrower. So now the money isn’t there to pay off the bonds when they mature, so you HAVE to refinance them with another bond issue to pay off the original one. Except now the aircraft aren’t new and shiny, they are five (or ten) year old USED aircraft, and that makes their market (and collateral) value less and even at that, the used aircraft market is flooded and so you wind up needing to sell the new bonds at a higher coupon rate.
which increases your debt service cost without decreasing your debt.