However, there are still way too many people who reach Steps of Faith and can't clear it because they've been carried through all content (and yes, I say STILL because the other day a friend was stuck in one run where they got Vishap to 50% by the time they got to last gate).
Yeah, on the DF. This proposed change wouldn't apply to DF, only to premades.

Oh also the undersized party option was not a "thing in ARR that disappeared with HW." It was added post HW release to ARR instances
Undersized was definitely a thing before 3.0. If I'm remembering right, what it didn't allow you to do at first was actually queue "undersized" with a full party anyway.