As to Millennials' tastes driving the art market going forward, here's what I wrote 3 years ago:
I believe the value of Lee UXM/XM OA (and OA from other popular early-‘90s runs) will appreciate in the coming decades. Present day 30-something year old collectors who were in their formative childhood/adolescent years when they read these comic books in the early-‘90s, are now hitting their prime income-earning years. As nostalgia for childhood pursuits kicks in and their spending power grows, this group of collectors will spend increasing amounts on OA (and other collectibles) from the 1990’s. This demographic also represents the last time that there was a critical mass of young comic book readers. From the late-‘90s onwards, youths tended to favour electronic forms of entertainment, and comic books increasingly targeted adolescents and adults.
[https://comicbookinvest.com/2017/01/13/artist-spotlight-jim-lee/]
And the meteoric price jumps that late-'80s/early-'90s OA has since undergone bears this out