How far will corporate lobbyists go to defend, not an industry, but
the particular business model an industry happens to be using at the
moment?

Obviously, I'm hinting at the efforts of the Recording Industry
Association of America (RIAA) and the Motion Picture Association of
America (MPAA) to shut down file sharing on the net, arguing that the
sale of songs and movies as if they were physical objects, and not
easily copied bit streams, is the only way vibrant music and film
culture can exist.  It's easy to conclude that the net must be a
serious threat to the industry's existence.  Surely if there were
feasible ways to adapt and modernize, they wouldn't fight so hard.

I doubt it.  Jump to another powerful industry:

A story in the December 25, 2005 Oregonian details how an April 2004
Oklahoma law cut seizures of illegal methamphetamine labs by 40% in
five months (and later by 80%), by restricting the sale of cold
medicines based on pseudoephedrine to licensed pharmacies and
requiring customers to sign a logbook.  (A friend of mine teaches at a
rural public school and tells heartbreaking stories about the toll
taken on her early-grade-school age students by meth abuse in their
families.)  Naturally, other states wanted to emulate that success;
writes the Oregonian:

  Officials from a group of of about 20 states began exchanging
  e-mails and meeting in person to draft their own pseudophedrine
  bills.  The group remained in frequent contact throughout 2005.

  "A lot of it is quarterbacking how to deal with the damned lobby,"
  said Iowa's grug czar, Marvin Van Haaften, a former sheriff.

That's right: the pharmaceutical industry fought the law because their
customers would need to find a pharmacy and sign a log book.  Roll
that around in your head a bit, but that's not the punch line.

  As [2005] began, Pfizer Inc. said it would reformulate the
  best-known cold medicine on the market, Sudafed.  Phenylephrine,
  which cannot be converted to meth, would replace pseudoephedrine.
  In one bold move, the company tore a hole in the industry's argument
  that pseudophedrine was indispensable.

Let me say that again: pharmaceutical industry lobbyists fought laws
known to be effective in reducing local meth production, even though
there was an alternative formulation for the medicine, known at the
time, that could not be used to make meth.  There's your punch line:
they would rather have the local meth labs continue to function than
change their business processes.

So, when you read press releases from the RIAA and MPAA, remember that
there is no humane, let alone reasonable, limit to the price lobbyists
are willing to ask the public to pay to allow industry to continue
doing business the way they do today.

According to Billboard Magazine, of a $17 CD, the record label takes
only 59 cents in profits.  $9.57 goes to retail markup and "Company
overhead, distribution, and shipping", costs that would disappear if
music were distributed digitally, directly.  $2.15 go to marketing and
promotion, activities which are supposed to pay for themselves.  Tell
me again, how hard would it really be to find a business model that
lets bits be bits, but preserves a 3% profit?