This is why they implemented Party Finder, so that people could find and form groups to do specific content with like-minded people.

As for them implementing difficulty scaling for dungeons and raid content, I think you provided the answer for why they will most likely not add it, it is a monumental task that would require them to go back and create multiple difficulty/balance variations to accommodate for this and they most likely feel that such time and effort is better spent creating new content.

Although being able to queue for df content with an incomplete group would be kind of neat just to see if you could clear content without a second dps or no tank or whatever.