Younger Millenial/35 and underer here. 32 this fall, actually. I think it makes perfect sense. To my generation, if we don't know/care who these characters are... It doesn't matter. You take away the cartoon nostalgia and this is still D&D. I don't know who Venger, Skylla, and Warduke are but that's very clearly a powerful Lich, a badass witch, and a super spooky death knight on the new DMG.
I don't think any young person who found D&D through Critical Role, who then goes to an LGS or Target to get the books will look at this cover, and think "Oh wow. This art is really cool" but then find out that it's all old cartoon characters and have their mind changed. The growth of the OSR has shown us that there is most certainly a longing for "The Good Old Days" and I think this is a way to play to that crowd, without disenfranchising the people not in that crowd.
So two weekends ago I went to my LGS to draft the Baldur's Gate Magic the Gathering set in celebration of D&D's 50th anniversary and while I was browsing the store waiting to see if enough people showed up for the event to fire off I overhead two younger(Relative to me. I'm guessing they were in their early-mid 20's) guys there, waiting for Adventurer's League to start, and talking about the new edition, and they got onto a tangent about 4e. By the sound of things, they had never played the edition and were dogging on it pretty hard with the usual criticisms. (It's WoW the TTRPG, Everyone had video game powers, etc) The guy working the counter, who is also in their age group mentioned that he didn't know anyone who had, or does play 4e. I mentioned that it's seemingly going through a bit of a renaissance, at least in online spaces.
They were really surprised about this. It seems to be another one of those things that you're probably not aware of if you're not chronically online. I think the general consensus is still that it was the Wii-U, or New Coke of D&D editions.
Hey now.. Watch what you say about my para-social older brother figure. I've spent a lot of time deifying him and I won't have you upset that pedestal.