Because the data present in the decommissioning nodes would eventually be transferred over to the live nodes. Is this understanding correct?
The replicas are not invalidated on decommissioning nodes even after replicating, so the capacity tracking was not accurate either. It ended up double counting the used space toward the end, at which the process seems to stall more frequently nowadays (this is another topic). If a significant portion of a cluster is decommissioned, the stat will look very strange and confuse people. That actually happened to us multiple times. The free/total ratio will look considerably smaller than the actual value. Monitoring tools cannot easily dismiss it as 'Nah.. it's a temporary discrepancy caused by decommissioning.'
With this change, the storage capacity stat has become more like regular under-replication scenario caused by node/disk outages. Additional space will be used for re-replicating those blocks, but it is not yet allocated to those blocks. That's the actual state of used/usable storage and the stat reflects that now. If we want the stat to reflect what would be used in the future, we are talking space reservation feature.