Lego Death Star will probably have 15 years of shelf space... I'd argue that the update doesn't count as a separate set, they mostly did cosmetic changes in order to be able to increase the price a bit.
And to me, MF is more iconic than the death star (if only because it's in all the movies, not from a single one, and thus will be seen in upcoming movies).
At the very least, I can't see the Han Solo movie not including the MF, and I can't see Lego ending production of MF before the Han Solo movie...
(Though I'll fail for the trap, I'll probably order it quickly in the really unlikely case it is indeed a short run...)
Edit: if it's over $600, I can see it not being a hot seller... Even for a legendary set, there's pushing things too far.
Edit again: I can see the "short run" rumors being engineered by scalpers, there's a strong possibility that the first run sells out quickly, and if Lego doesn't communicate well, there's a strong chance at quick cash, even more than with consoles (or Amiibos
)