do you think lemmy will ever be popular?

I'm gonna say yes, for the exercise.

Four assumptions:

1. Reddit will keep getting worse, due to the nature of enshittification and venture capital. Eventually enshittification reaches a breaking point where people leave or stop arriving.
2. Lemmy (in a broad sense - et al!) will keep getting better, due to.the nature of open source software.
3. Non-free alternatives to Reddit will eventually enshittify, law of enshittification.
4. Free alternatives will use ActivityPub for the obvious advantages.

If these assumptions are met, given infinite rounds of enshittification and unhappy users, eventually a federated and free alternative will be the most lucrative option for the majority of users. Eventually Reddit will Digg itself a hole. Maybe Lemmy won't take over then, but it'll stick around.

The most unrealistic assumption is of course that the federated solutions will keep getting better indefinitely. Maybe they won't. But as long as people keep developing and contributing to the Fediverse, it's alive and improving in a way commercial alternatives cannot in the long run compete with.