Earlier quoted context omitted.
How could it replace Twitter? Even the Mastodon devotees admit it can't scale to that level and the sign-up process will turn off most users before they even get started.
Maybe I could pose this question another way, what does Twitter do that Mastodon can't?
For now, Twitter scales better. There's no such thing as an instance, a slow instance, or registration simply being closed.
Twitter preserves content. Yes, I know it's currently debatable, but in general it does. On Mastodon, any instance can go the way of the dodo, and this regularly happens. Further, it's standard policy to wipe out all media attached to your toots. I think this point is a big deal. Your content simply isn't safe on Mastodon.
Mastodon has poor on-boarding. You need to pick an instance which a normie doesn't understand. Further, it has no (good) algorithms for recommended content, followers, finding people you already know. You have to bolt and stitch things together. This too is a massive issue in the age of Tiktok, where even the effort of lifting a finger is too much. I think organically building up your feed is actually great, but you have to understand that the HN audience is not the same as the masses.
Mastodon has no consistent moderation. Yes, you could argue neither has Twitter, but it's still consistent-enough compared to Mastodon, where you're at the helm of whichever volunteer runs the instance. Not necessarily an issue for middle-of-the-road conversations, but still a difference.
Lastly, and this is perhaps the most twisted point: Mastodon isn't designed for drama. Twitter is hyper optimized for it. It's quite the culture shock for some.