One unknown element for all airlines with high levels of debt is what happens to interest rates in the future. As long as the global rates remain low and assuming business returns quickly they can pay down the debt and be fine.
If rates climb and business is slower to return then some may face the prospect of refinancing at higher rates. That's when things become challenging.
TUI have a wide base of operations across Europe so I imagine they will be better placed than some others. Time will tell but I wish them all well.