D&D's Spelljammer and Planescape reimagined/reworked/updated as "campaign collections" for 5e: Cynical nostalgia-bait for middle-aged tabletop fans, or valid bridge by which a new generation of fans can discover with mountains of old material?
probably both! Personally I could do without “hey remember the characters from the 90s-2000s crpgs? They’re on the books now”. Just advertise the setting on its own merits!
I do think that there are enough dedicated players who are curious about older editions that they’ll seek out the older material. I hope they do! That was me when I started playing (first experience was 3.5e, first full campaign was, unfortunately, 4e lol)
Now, is it nostalgia-bait to hire Tony DiTerlizzi to do the new DM screen for Planescape 5e? Maybe, but also they should’ve given that guy a gazillion dollars and let him cook for the whole set.