Nobody mentioned cost so far, nor Windoze....
It is a long time since I was involved in part of this, but I was involved :-)
To me, MIME was a commercial enabler, and a cross platform enabler.
In the '90s most offices (those not on VMS and dumb terminal systems) ran WIndows of some sort.
So they could easily exchange complex attachments because Windows understood what they were - due mainly to file name extensions.
So each business (or part business) had a network and some mail servers.
ccMail, MsMail, Novell, Lotus Notes (whatever happened to that?) and a few others.
Within the networks they could exchange documents fine - it was just sending outside the organisation was a problem.
The main problem was cost.
There were plenty of email gateways - but for each one you had to licence another email system and the licence fees were not cheap!
SMTP provided email interaction without licencing the protocol.
MIME enabled rich information about attachments to be included.
These two things freed email users from being tied into one supplier by large licence fees.
Microsoft Exchange Server killed off the prorietary email systems by the usual Microsoft tactic of finally getting it right about 10 years after several other firms had fought for market domination by being better than Microsoft in a chosen field (much like IE finally killing off Netscape, Novell withering once M$ networking finally got its act together).
M$ tends to lose all the early battles but win the war.
So really MIME was not necessary for medium to large corporates because they already had solutions in place.
It did however make inter-corporate working easier and cheaper.
The real impact was for non-corporate users and non-Windows users.
Having a standard which was not tied to major commercial email suppliers and not tied to Windows file extensions made email effectively supplier/vendor/OS independant.
Which is nice :-)
Finally, X.400 could do all the transporting of complex document types to and from all kinds of different devices in all kinds of different languages and character sets - essentially everything that MIME could do (possibly more).
However it was so analy complex and difficult to implement that PCs couldn't hack it in the early years - just not enough power.
SMTP was simple by comparison, although feature poor at the start, and could at a pinch be worked from a teletype connected directly to a port on the mail server.
Just my own take - back through the mists of time.