In general, I agree that pro/rel is not necessary. It's just a name sake, a different way to classify and divide. The only area where you and I disagree is on youth development. I still argue that pro-rel (assuming that there is a method to enable upward mobility of players - i.e., there's a low barrier to entry from second division into the first, particularly for younger players) is a requirement for quality control. This is only assuming that there are no other competing leagues, such as college or B team leagues.
Promotion/Relegation is crucial for youth development and development as a whole. Bigger clubs with promising young players need playing time and they can loan them out to low echelon teams in the Classic or the Challenge, and it keeps the talent from top to bottom in check, allowing the top players to continue to play against the best competition as possible while the developing players can fail and fail until they get better.
Why is it crucial? I gave some real world examples where nations with pro/rel compete directly with those that don't, yet the leagues with pro/rel aren't producing the better players. I'd also argue that something like the Major/Minor system in the US, coupled with the type of national curriculum that's a hallmark of Dutch youth development and a cornerstone to Germany, Belgium and Uruguay's recent overhauls, does an excellent job of placing players at the appropriate levels. Good players can move up and down the leagues, while the minor league teams can prioritise actually developing the players. Meanwhile in pro/rel leagues, a regular theme is the reluctance of teams struggling against relegation to turn to their youth. Instead, short term measures become the focus, often leading to the signing of journeyman who are unspectacular but reliable. The need for quick results also means that the long term development of players is often not a priority to a coach, who is often more fixated on getting the immediate wins that will keep them in a job, rather than a prospect coming to fruition in 3-4 years time.
I think you make good points. I didn't read your original post. However, I'm going to take a guess and say that the nations you picked for leagues without pro/rel system is because they don't need it, due to the fact that they have been producing players and have been a good footballing nation for a very long time. This isn't the case for US or Korea where they're not a powerhouse.
The US does have the skeleton of a good framework for targeted youth development. However, the component parts all have their own issues. Then there are some legal issues. The NCAA has a heavy overlap with the amateur and semi-pro leagues and its various rules that govern student eligibility cause complications. Then there's the Fraser vs MLS lawsuit agreement which pretty much prohibits US teams from benefiting from FIFA's training compensation mechanisms as it doesn't allow teams to receive fees for players they don't actively have under contract. Let me put this to you: Korea aren't a powerhouse, but they have traditionally been very dominant in AFC, despite previously using closed leagues. In fact, when you consider that A-League, K-League and MLS have all won continental titles at some point, though there are certainly caveats, closed leagues don't seem to be holding clubs back particularly. That could be circumstantial of course, with none of the three playing in UEFA or CONMEBOL while also being strong economic powers in their own right.
Pro/rel is not a necessity for success, but it's more beneficial than harmful in general. It doesn't have to be, but my guess is that pro/rel generally creates more competitive lower divisions. Playing against teams that were just recently in the top flight is better competition than playing against teams that have no history or hope of doing so. It also gives players (and teams) a better sense of where they stand up to compared to top flight teams, which can be very helpful as a player decides how to handle his development. And it incentivizes club managements to build more competitive teams in the hopes of reaching a higher division. I don't think your solution of letting lower division sides prioritize youth development entirely makes sense. First, these teams are competitive teams, not glorified reserve sides. Having lower league teams focus on development at the cost of other objectives that a team might have (e.g. winning) contradicts the entire purpose of the league. Might as well drop the farce and pump more money into the reserves league. Second, your solution assumes that players can freely transfer between the leagues. In theory that sounds fine, but, while I don't know about the US, in practice that doesn't seem to quite hold. I can't be certain, but K2 clubs seem to do a lot more business with each other and K1 clubs than with National League (the 3rd division, with no promotion to the second). In England, how many times do PL clubs buy promising Championship players, compared to PL clubs buying promising Championship players with some PL experience? A young player whose team is promoted has a much higher chance of coming across the radars of PL clubs. And PL clubs can see how he stacks up against other PL teams. In a closed system, that player would be harder to pick out of relative obscurity, and prospective employers would have a harder time verifying his quality. Lastly, you point to several examples of countries without pro/rel that did fairly well. The question isn't whether "closed" countries did well. The question should be whether these countries could've done even better with pro/rel. It's entirely possible that the lack of pro/rel was a hindrance, but that other factors outweighed it. (And here it's not hard to find examples of why Aus/Jpn/Kor would've outpaced other countries...) As an aside, I can't really agree that the US has a "good framework for youth development". College drafts are a big hindrance on the development of US players compared to other countries.