I spent years writing about the weakening of agency skill challenges represented, precisely because there are so few tools available for players to force their desired outcomes, and when 5e came on the scene, I saw it's skill model of 5 generic DCs as simply moving further in that direction. I don't see see how setting the number of skill checks to get to a negotiated outcome is a sea change over negotiating check by check, or after an unknown number. The actual player levers to pull are still sharply limited.
I don't think those are the ends of the spectrum. If anything, they feel pretty close to one end, with an objective, action-by-action specified, skill system on the other.
To be fair, the "convince the Baron to help you" or whatever it is in the 4e DMG is probably the weakest example SC WOTC produced by far, and most counter productive.
However, I just cannot comprehend (given an open-skill SC using the Skills Compendium or DMG..2? rules) how "player makes a fictional statement and chooses a skill to progress towards their clearly defined goal" is lower in agency around goals of play unless we're back at the "I have more agency if I don't have any outside the character knowledge" stuff. Maybe it's because all the examples of SCs I've seen are the stuff @pemerton and ManBearCat ran for me where it's a mechanism to do Story Now play in D&D in a super satisfying manner.