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.