It doesn't need to "replace" Reddit. As long as Lemmy becomes large enough on its own it can sustain itself, Reddit doesn't need to die for that. Reddit and Lemmy can coexist IMO
It has already replaced reddit for a lot of people. It would be difficult to surpass it, but after all it doesn’t matter how many users a social media has if most of them are lurkers. From what I’ve seen on Lemmy, people are more willing to engage with others. I’m one of those people, in the two years I was on reddit I didn’t make a single comment or post. Maybe it’s because the fediverse is much smaller than reddit and feels more like a community. In that case I wouldn’t hurry to “replace” reddit.
Personaly: already has.
In general: I hope not because that would mean I need to close my own instance (and every other instance as well… server cost for Reddit is running in the millions per month!)
I left slashdot for reddit, and reddit for lemmy. Those were replacements from my point of view.
I'm not sure what you mean with "replace" in general. Taking over their servers and replacing the actual running software? Or maybe using their domain name for a lemmy instance? Transfer all logins and data to a lemmy instance and delete them from their systems?
Those are unlikely.
Becoming mainstream, having more media attention and influence than reddit? Then becoming more interesting for spammers and infested with lower quality content? Hopefully not.
You mean the Fediverse as a whole, not any one specific platform? In that case yes. But it won't be just Lemmy.world or even all of the Lemmy instances. It will be all the platforms that use the ActivityPub protocol that in combination will replace Reddit.