I've heard that theory so many times and I think it's irrelevant. The number of would be SP only players is such a low percentage of the total... and how many of those would really spend money on the RMAH?
You're missing the point. It's not SP only people that would do it. I played a lot of multiplayer D2, and eventually wondered what it would be like if I actually had all the items for the builds that I wanted to try.
I downloaded the character editor and made single player character with all those items and tried it out. After that the mystery was gone and I really didn't have a desire to play all that much.
If there is no other way for me to try out a build than to actually get the items, it's going to take more time and effort or money.