From Steve Deering:
https://youtu.be/fvc0tHimVbo?t=9m58s
"""
Our experience looking at what the addressing plans that the OSI
community had come up with, they had specified this maximum of 20
and then they went out and designed plans that were exactly 20
bytes long.
So the nature was to basically provide lots of extra space in these
addresses and so I don't think we could have sort of kept short
addresses very long, the nature would be to just consume them and
make them longer.
"""
In practice, variable length addressing rapidly becomes maximum length addressing.