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Youtube is probably the most difficult platform on the internet to replace. Video content delivery is extremely resource heavy and technically complicated, especially once you start to scale. Many, many competitors have attempted it over the years, and while some found their niche, none of them have achieved anywhere close to the scale of Youtube.
It took decades of Youtube to become profitable, only doing so after achieving mind-boggling economy of scale. The majority of humans on earth have used Youtube. About half of all (global) internet users use it monthly. I don’t know if any other platform can claim stats like that.
Youtube is one of those platforms that only exists because it got a head start in the unique conditions of the early internet. I don’t know if it’s even possible to create a true competitor, though I could see multiple platforms taking over different niches.
I think most self-hosted Git+CI/CD platforms have container registry as a feature, but I’m not aware of a service that is just a standalone registry.