Because they will have handed back their address space. The address assignment policies are explicitly designed to have a garbage collection mechanism under 2007-01. If you don't actively maintain your registration, it reverts to the registry.
Nick
Sure, but consider this simplified scenario:
1. In June 2019, IXP A requests an allocation. They get a /24.
2. In June 2020, the reserved pool is exhausted.
3. In July 2020, IXP B requests allocation but can't get one due to 2.
4. Sometime in 2021 (or even later), IXP A hands over their allocation.
What would IXP B do in the mean time between July 2020 and 2021 (or even later) when IXP A handed back their allocation?
Of course another IXP could have handed back their address space, but this doesn't change the point made:
Defaulting to a smaller allocation increases the chance (if ever so slightly) that the pool will be exhausted at a
later point and thus IXP B will get an allocation in July 2020.
On a smaller note, I could also tentatively argue that being able to hand over new allocations is more important to ecosystem
diversity than being able to honor requests for increasing existing ones.
Kind regards,
Aris