So there's two limits to most commercial aircraft: one of the number of pressurisation cycles the airframe goes through, and the other is number of flight hours

So there's two limits to most commercial aircraft: one of the number of pressurisation cycles the airframe goes through, and the other is number of flight hours. Typically it's the former that's the limit run into in practice. Clearly the fact that fewer aircraft are flying increases the lifespan of those aircraft (assuming they aren't scrapped or inappropriately stored!). But that's often not the only concern the airlines have: some airlines pride themselves on having a fleet of relatively new aircraft, fuel is a significant cost and can be a significant incentive to replace aircraft with more efficient ones regardless of age (especially if you have competitors on the same routes with more efficient aircraft), time out of service due to maintenance (at some point having the aircraft out of service for a couple of months with a large bill due just isn't worth it versus replacement, and aircraft maintenance is mostly done on a time basis so the clock is still ticking). Delaying aircraft replacement by a couple of years due to 737 MAX delays is almost certainly viable, but delaying replacement by 5+ years to instead order A320neos is harder, and delaying replacement by 10+ years if you want to push Boeing to replace the 737 is likely implausible.