When "most people don't realize" something, the development team has a good opportunity to remedy the situation. Clearly you have the stick, why not offer a visible carrot? Give a buff to those who join in-progress dungeons: "you will earn a 10% valor bonus when the raid completes. Additionally, you will be given preferential placement in your next raid finder queue."
It might not satisfy the OP, or others in this thread, but it could help the overall attitude toward in-progress content.
Something like this could work.
Problem is, with most of these people, nothing short of a guaranteed one shot will work.
Pathetic really. This thread just shows how terrible this community can be.
I taught a group how to do LFR garalon when it first came out and was stupidly difficult, and endured 6+ wipes. If there was no chance of me getting an upgrade from the work I put in, I would never of done it. I spend hundreds of hours a year teaching people how to play this game, I'm not going to do it pro bono.