Cris Fitch wrote:

Not long ago it looked like the Medium lift market was
over-subscribed with Proton, Ariane-5, Sea Launch, Atlas-5
and Delta-IV. Now with the retirement of Shuttle and a
new plan for manned exploration coming into being, we've
got to ask ourselves:

1) Launch lots of medium payloads
or
2) Go Heavy

Option 2 would be a dream come true for whoever gets to build the heavy lift
vehicle. The barrier of entry for such a beast is very large, so you do not
have to fear competition that much. If you had a standard payload size of,
say, 5 metric tons, you would have many competitors from the start, and
even completely new launch methods such as space tethers, space elevators,
TSTO or SSTO space transports could enter the market at comparatively low
cost.

Given that, it would be a mistake to let the provider of the launch vehicle
design the payloads. That way boeing might have just enough "unexpected"
weight growth that their own vehicle is the only possible launch option...