It’s a wrong way to view it. It doesn’t have to reach 1 million active users on its own. The whole point of it being federated and using ActivityPub protocol is that it CAN be compatible with any other project that shares similar values. Lemmy is still very young and missing a lot of features, but the underlying protocol makes the data portable even if functionality is not built-in yet.

People also said Mastodon will be empty after the anger about Twitter will subside, but it managed to keep it ~1 million people after its peak of 1.8m. Fediverse should grow on its own pace and not be compared to for profit social media companies. Unlike for profit companies which need a big number of people to join for investors not to leave and keep investing to stay alive since they make 0 profit, Fediverse is self-sustaining.