Also, MySpace vs Facebook is a false equivalence. MySpace launched in August 2003. Facebook launched in February 2004. These two brands were built simultaneously. MySpace never had a prolonged period of unchallenged market dominance. They were founded almost simultaneously and then one pulled away from the other.
While these dates are technically true, it's not exactly the reality of the situation. Yes, they were founded at that time, but facebook wasn't public until about 4 years later. So Myspace had a solid 3-4 year lead. But even after it went public, it took an additional 3+ years for people to completely abandon the MySpace ship. It's not like facebook went public, and then MySpace was obsolete the next day. These things take time.
As far as YT goes, all it would take, is for the ad revenue to be better for the content creators. For the most part, YT is a free platform, and it's built by it's content creators and their audiences. They have a symbiotic relationship. If the content creators see that they can make 25%-50% more money on ads over at a new streaming site, they will go there, and their audience will follow. For over a year now, YT has hit content creators financially. Most people have seen around a 50% dip in their ad revenue, while also getting flagged, demonetized, or just outright censored. Again, if these creators see a platform that is similar to YT, but with ads that are paying them more, they'll move. Money talks.
And as long as Amazon can make enough apps for phones, tablets, video game consoles, and TVs, people will go where their favorite content creators are. You can already see this with YT and Twitch. I've seen plenty of audiences go to either platform, depending on where their favorite content creator is. I've seen popular video game YT'rs who had a massive audience make a switch to Twitch, where they saw an exponentially growth, while they abandoned their YT channel. So we're already seeing it happen a little bit with video game YT'rs. And for people who do both YT and Twitch, I can just see this being even more of a reason to stay at the AmazonTube site, as it'll most likely be tied to Twitch's site, as well(Amazon owns Twitch).
So who knows, we'll see. Either way, nothing will happen overnight. We'll just have to wait and see if this even comes to fruition.