I'm not even sure why people need to argue about this. If you want to use Mastodon, then do it, and if you don't want to use Mastodon, then don't. Is anyone forcing you? It's a personal choice. And no, Mastodon will never replace Twitter for hundreds of millions of people; it was never designed to do so. Mastodon vs. Twitter is a false dichotomy. It's more like Twitter vs. itself, Twitter vs. not-Twitter. From my own perspective, Mastodon is simply a place I happened to go after I already decided to stop using Twitter.
I suspect that to a large extent, arguing Twitter vs. Mastodon is only a proxy for arguing for or against the new owner of Twitter, and this argument has been rehashed ad infinitum on HN from various angles.