Sorry, misunderstood when you said "Starting an airline is not a trivial undertaking, certainly not something you could hope to complete during a temporary downturn or use "market timing" like you would with equities.". I thought you were implying the founders of Norwegian spotted an opportunity and started-up during the downturn.
Outside of some state backed airlines, I would have thought any airline founded in the last 35 years will have started with 100% leased aircraft. For airlines that own their own equipment, in the first world at least, most of them tend to offload aircraft at the 17 to ~20 year stage (at which point they're usually sold on to leasing companies who then lease them out to charter/acmi operators or they find their way down to Africa or LATAM).
The problem with buying your own equipment (from a budget airline PoV) is that you can't benefit from newer model fuel efficiencies or improved pax capacity. They also put much higher pressure cycle loads on the hulls due to the increased rotation frequency of the short-haul point-to-point model (which reduces the serviceable life of the airframe and hence the re-sale value). Running a mixed leased/purchased fleet starts making more sense when your route network isn't dominated by short-haul.
As an example, Ryanair's average fleet age is < 6 years (across 300 aircraft).