There really are no viable, slipstream alternatives. The only entities with the resources to spin up a massive, centralized social link aggregator and community-based discussion system would be a handful of companies in big tech (Facebook, Twitter, Alphabet/Google, etc.), though none of them have platforms that are appropriately analogous to reddit. Even if they did, three weeks to migrate and onboard millions of users is a tall order.

Lemmy is the closest thing I've found so far in terms of a similar experience and UX, and while it's still pretty rough around the edges (mainly in terms of UX and infrastructure redundancy), the decentralized nature enables it to scale horizontally without requiring resource expansion for a single player. It definitely needs some work to optimize instance implementation and capacity-based promotion, but I believe it has a lot of potential.

With that attitude it won't be.