Yes, a series of body parts with Content-Disposition: Inline
could give a similar effect. But why is it better than HTML?
Because it gives you the pretty pictures without exposing you to the
risk of active HTML content.
Few existing mailers give good support for producing a series
of body parts. You could of course try to get mail software
developers to support sending of a series of body parts
with Content-Disposition: Inline better. But will you succeed?
Given enough education? Probably. To have a realistic chance
though we would need volunteers to fix this in a couple of
popular open-source GUI mail clients in order to expose the
concept to the masses.
Another usage of a series of body parts, which I think would
be very natural, would be to include the message you are
replying to in one body part, and putting your reply in another
body part. But this, also, has not been accepted by the market.
Do you mean:
text/plain (my reply to the message)
message/rfc822 (what I am replying to)
or are you talking about intermixing quoted text with reply text
(as I'm doing here), but with individual MIME parts for each
chunk of quoted text and reply?
If the former, it hasn't been accepted because it almost always
presents the quoted text in a useless manner. Appending a large
blob like that doesn't give any context to the reply, and without
the context, the quoted text serves no useful purpose.
The Plan 9 MUA (the one built on acme) appends the message being
replied to as a message/rfc822, and IMO doing so is just a waste of
disk space and bandwidth.