Because there are old messages, all of the existing methods need to
continue to be supported indefinitely so that those old messages can
still be read.
Analogy: because there is still data on old 5.25" floppy discs, all
computers need to be able to read them indefinitely.
Of course not. If you want to read an old 5.25" floppy, you find a
conversion service.
*Something* needs to be able to read old messages indefinitely, but I
don't see why it has to be *everything*.
and note that I didn't say 'indefinitely'. but lots of people keep old mail
around. while it's hard to argue that people should keep buying 5.25" disk
drives for the sake of reading their old floppies (partially because the
medium is essentially useless for storage of new things), it's a bit easier
to argue that MUAs should contain a bit of extra code to handle slight
differences in message format that were once widely used.