What's the downside to having 128 bits over 64? I'm not familiar enough with file systems to know, so I'm left wondering if there's any downside to oversizing the hammer for the nail?
It expands the data structure size, which multiplies N for both on disk and in RAM representation (and cache and shorn cache lines). Many modern architectures have native 128b types (amd64, POWER) so it's not really a big deal to the CPU itself even if you need to do atomic operations for concurrency.
OTOH it guarantees there wont really need to be on disk changes for pointer sizes. That may be useful in situations that weren't really imagined, for instance 128b is enough for a Single Level Store where persistent and working set (or NV) are all in the same mappable space for any conceivable workload.
tl;dr: doubling size of metadata (and usually a half of a 128bit address is zeros anyway).
Having twice as much space occupied has many implications: twice as much space lost for data itself, less metadata will fit CPU caches, less throughput in buses between CPU and storage, lengthier read time for the storage itself, etc.
