]Since "Faster better cheaper" is the cornerstone if affordable manned space flight, their resistance to the idea should tell you something.
It tells me that folks are fed up with the Dan Golden era stagnation of endless also-ran Delta II missions of the Goldin era. And six or seven Delta IV heavies will easily cost as much as a single production SLS launch--thus no savings--just overcomplicated ISS style assembly methods and other costs that go with mission complexity that HLVs eliminate. That's why engineers favor them. Take the skycrane for Curiosity. That work-around is the direct result of EELV contraints.
Even the Orion capsule is probably going to perform its first lunar flyby off the top of a Delta-IV Heavy.
It's about the fact that HLVs are unneccesary for 90% of what we want to do in space
On the second bit, SLS is directly necessary for what we want to do in space
The first Delta IV flight won't be a flyby but a simple test to a high Earth orbit to test the heatshield.
Boil-off problems are not solved, and any depots are likely to be SLS launched anyway:
Also remember that the larger an LV is, the greater the internal volume growth--External surface area grows with the square with volume increasing by the cube--so having large diameters, especially for hydrogen--should be encouraged:
As it stands, we are well on our way to SLS
On D-IV vs Falcon heavy
In terms of a down select--it would actually be better if Dream Chaser and Musk got the contracts, perhaps allowing them to work together and pool money, instead of Dream Chaser being a junior partner to ULA