Their data is what is valuable to users. Their users is what’s valuable to the business. Supply and demand my friends. If you delete the supply, there’s no demand.
The demand for information and entertainment is independent from Reddit supplying it, yes. But if everyone used shreddid, this demand could no longer be fulfilled on the Reddit platform, thus the demand for Reddit would cease.
This is cutting off one's nose to spite their face. It's not like it'll be replaced by a new trove of historical conversations going back over a decade. That's still valuable to people not named Reddit.