Reddit does still have the advantage of being highly indexed by search engines.
A Lemmy instance on the other hand probably wouldn’t have that same benefit.
How often do people search for “how to [insert thing here]. Reddit” to find a worthwhile source of info that isn’t clickbait?
Unless the communities you visit move, it’d be very difficult to move entirely.
This applies to discord especially.
They’re more of a compliment to the other social networks in many cases