Soun dying along with Kasumi wasn't meant as a copout.  The point is that
Kasumi and Soun are symbiotic -- one can't exist without the other (hence the
ghost's remarks at the end of Kasumi's dream).  The two deaths are also meant
to facilitate revelations of Nabiki's and Akane's true character and
instigate a new phase in their lives.  Kasumi's and Soun's passing represent
the passing of the old.  Also, Kasumi's death wasn't meant to be a suicide,
in spite of her depression.  Instead, she is simply a character who lacks an
identity, and, being unable to find it, is destroyed for her inability to
exist in the real world.  The family is Kasumi, and Kasumi is the foil of
Soun.  The substance of both characters died long ago with Kasumi's mother.
 At this point, Kasumi and her father become merely tools to facilitate
Nabiki's and Akane's growing up.  Once Akane and Nabiki mature, then there is
no need for Kasumi or Soun, nor is there any purpose in the lives of either,
hence their simultaneous deaths.  The story then establishes its real focus:
Akane, Nabiki, and their initiation.  Kasumi's suicide note is not so much a
revelation of Kasumi as it is of Akane and Nabiki both, the former by her
reaction and Nabiki for forging it (which will later be revealed once I post
the rest).