First of all end-users would not care much what protocol it is when
"you've got mail" pops up.
No, but they might care about having email work more reliably.
And for designers, programmers, and quicker implementation its quite a
bit
better if new protocol and old one share certain components. For
example
MIME encoding/decoding, etc. It would faciliate process if protocol
can
be implemented on same mail server software that would be able to
decide
based on certain dns or other parameters what protocol to use when
sending
email to the other end.
Yes that's why I would consider running both protocols on the same port.
On the other hand if the new protocol were designed well then there
would be less need to share components. For instance if all mail were
binary transparent and delivered directly from the sender's submission
server to the recipient's message store (which is close to what you
need for reliable error reporting) there would be not be as much need
for such an MTA to encode or decode MIME.
And look at the example of IPv4 vs IPv6 - IPv6. While there are
substantial
improvements in features available with IPv6, as far as implementation
and
design and application programmings, its not that different.
Not at first glance. But for a variety of reasons (e.g. address
scoping and new kinds of hosts) I suspect we'll end up programming
differently in IPv6 than in IPv4.