> This is perhaps a naive question, but are there many machines that need more than 64bits of address space?

Not many machines currently, but CHERI has 129-bit pointers (yes, 129), and there's already some work on for instance Rust to be able to work with that (see https://faultlore.com/blah/fix-rust-pointers/ and https://faultlore.com/blah/tower-of-weakenings/), and also on GCC and glibc (https://lwn.net/Articles/909265/), and it's not completely improbable that this becomes more common. Creating a brand-new API today which cannot handle pointers wider than 64 bits seems a bit short-sighted.