I think there are two reasons.
First, I have no problem with the new behavior if it didn't cause a
regression. I am not sure about the history of other filesystems, but NFS
has had the old behavior for ages, and people get used to it.
Second, NFS is actually special as this particular setup is very common and
you'll get into this situation far too easily, as from the server you could
export two directories within a filesystem as if they were two filesystems.
Very few people actually want to mount the same local filesystem multiple
times, but under NFS this is the norm.
Last but not the least, NFS is often controlled by central corporate
policies (autofs/nis), and has to work with various clients. For example,
it's not possible to add "nosharecache" to auto.auto as almost nobody
understands it, unless you upgrade all the clients.
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