I don’t think any platform collapses overnight. What you have to do is do is make something “better” and engage in a campaign of attrition to get people to move over. Produce content that other visitors see and like. Submit links to that content to aggregators (e.g. Slashdot / Fark etc.). Even start submitting links on Reddit that lead over to Lemmy and so on.

Make Lemmy feel as normal as Reddit. People will get used to the interface, the quirks and perhaps stay. Every person who stays is one less for Reddit. Now “better” is doing some heavy lifting since Lemmy has some advantages (ad free, federated) and some disadvantages (inline media & limits, sign up confusion, app). The disadvantages need to be addressed and the advantages need to be made stronger.