That's a very strange reading of what they've said on the subject, it seems as if the long term work and planning that they're doing for this would be somewhat easier were they going to put out a coffee table art book and open it up on DMsGuild. Like, where did you even get that from? the MTG art books? Seems kind of a random thing to pull in. If they were doing that, there would be no reason to discuss their thinking on how to properly do Spelljammer or Greyhawk, they mentioned that the next step in the design process for the new classes was to filter them through Dark Sun and Eberron to make sure that they fit the settings they need to. They also mentioned that they attempt to seed new content in their previous content (Strahd's entry in the MM foreshadowed COS, for instance, Acerak is featured on the DMG, and so forth) Mike Mearls further invokes this here:
https://www.sageadvice.eu/2017/02/15/which-year-is-5e-spelljammer/ suggesting that Volo's is hinting at a Spelljammer adventure.
It also seems like part of the reason we haven't seen settings so far has to do with the release schedule, its been brought up that they expect 5e to last at least a decade, which we're still in the first half of- it's possible that they'll eventually move on from the Realms and do the next setting for a year or two, then move on again. I'm also not sure what sort of product you think they recently started testing on UA, we've gotten Eladrin, Gith, and Tiefling subraces associated with specific rulers of the nine hells, that has planar adventures written all over it.