This sort of dogmatic rejection of HTML ended up being the bigger problem. If instead futilely fighting against HTML email the community would have just embraced the idea, we could have a sensible spec for email (instead of just ad-hoc whatever Outlook does) and email could have had a chance to prevent its fading relevancy.
But no, we had to fight the great evil of rich text which left vendors, MS in the forefront, to their own means to fulfill the strong and justified user demand, with predictable outcome.
RFC 1896 defined a sensible alternative in 1996. I agree that it should have received more support, although at least a range of Unix MUAs and Apple Mail did and do support it.
But no, we had to fight the great evil of rich text which left vendors, MS in the forefront, to their own means to fulfill the strong and justified user demand, with predictable outcome.