I think they don't want to risk being tricked into settling for what might qualify in hindsight as "small change" -- such as one huge lump sum,
paid by DC, on condition that the heirs go away and never bother DC again after cashing a very large check -- nor do they want a guarantee of a smaller fixed sum
each year, also paid on condition that the heirs go away and never bother DC again as long as those checks keep coming.
I think what they want is a percentage
-- so that, for instance, if Superman-related products make five hundred million dollars in one calendar year, and then a new movie comes out and Superman-related products (including ticket sales, DVD sales, etc.) make a billion dollars the following year, the heirs would get twice as much
in the second year as they had gotten in the first! Instead of being stuck with a rigid number that had been nailed down in a contract before the new movie came along!