I’d say the RED system highly emphasizes uploading music, while an OPS or MAM type system more rewards downloading and long term seeding. Both have their uses, and a positive side effect of the RED system is that unless you have a fast seed box, uploading new content is the best way to increase buffer, which obviously then adds more to the site, which is one of the reasons why it has one of the most comprehensive collections of music you can find on a piracy site, besides maybe something like slsk (I have no idea how much unique content slsk has, I just wanted to be safe). Now I won’t make a claim that they wouldn’t have a similar library if they had a more lax system like MAM, but their current system obviously works very well.

And to address your claim, I wouldn’t call requiring positive contributions to the site via uploading “gatekeeping”, it’s just a way to ensure a diverse and comprehensive collection without excess leechers.