An "open" course becomes self-regulating based on the learner's ability, but may be weak in assessing results.
With iTunes U, Udemy, etc - yes you CAN skip ahead to lesson 20, but if you don't understand it the natural response is to backtrack to earlier lessons to fill in what you missed. The result is that novice learners follow a sequential learning path anyway, while advanced or curious "big picture" learners may explore more freely.
This meets the criteria of "understand A before doing B," but places the burden on the student rather than the system. There may be quizzes & assignments, but often no grades or validation. If valid assessment results are important to you, then a fully open system may not be ideal.
A fully "closed" course can become more about compliance and tracking requirements than learning or motivation. Possibly self-defeating.
What comes to mind w/an enforced sequential course is awful mandatory workplace training. Can't bypass the bad Flash animations & cheesy videos, can't avoid quizzes, and can't work at your own pace even if you already possess the knowledge. This approach assumes everyone is a novice, which is frustrating, and limits the content w/o regard for learner skill or preferences.
For the user this often leads to boredom and checking the boxes as quickly as possible to get it over with. For organizations the goal becomes more to have a record of "training" people on whatever they were just force-fed. Learning is secondary to compliance w/the rules.
Semi-closed option allows for "unlocking" & exploration while maintaining assessments to validate results.
Duolingo, your example of a "closed" course, isn't fully "closed".
Yes, future content is locked, but there's always an option to test out of modules by passing a skills test. Future modules only remain locked until you pass the assessment, whether you've reviewed the lesson content or not. Similar to the "open" model, this allows learners to skip ahead to an appropriate level if they have prior knowledge, while leaving all the unlocked content available for review as necessary.
This also meets the criteria of "understand A before doing B," but adds a system-driven assessment that both validates learning and augments the "gamified" unlocking/motivational aspect. This also meets the "paper-trail" requirements of a closed system w/o treating every user like a clueless n00b.
This approach is only "closed" up to the level of prior knowledge, so it allows for working at your own pace & exploring the content. Course developers can still require that all assessments be passed for completion, but the order and speed is up to the learner. If valid assessments are important to you, then this semi-closed pre-test/skills assessment model may be the best of both worlds.