There are limits on what you can do with HTML and HTTP. Being able
to access the archived message from IMAP is potentially a lot better,
but (as with many things) even IMAP needs some tweaking to make it
work well for this.
How many people actually use IMAP? (I don't.)
why does it matter what they use to read their mail, as long as the
right thing happens when they click on an IMAP URI?
lots of MUAs do support IMAP. whether they support the ability to
reference messages via IMAP URLs is a different question.
mail archives won't get better without specifications for how to make
them better.