Doesn't the current process assume that xz always produces the
same
output?
Yes, the deltarpm process depends on xz giving the same output
when run by the consumer of the .drpm as when run by the producer.
If not, then deltarpm gives a warning and ignores the .drpm --
the entire new .rpm must be downloaded.
What would happen if a newer version of xz/lzma comes out which (for
example) produces better compressed output while still remaining
compatible with the file format and older decompression tools?
Then there would be a "one-time" event where none of the .drpm
could be used, until xz on the consumer machine matches (catches up to)
xz on the producer machine.