Anyone that is interested in discussions on either defederated community, will create an account for both.
And that is the reason why reddit is still growing. If you are required to make multiple accounts just to engage with the communities you want to engage with, Lemmy is no better than separate forums. And those all got overshadowed by reddit for a reason.
There is an interesting op-ed adressing the ‘issue’ of ‘lazy users’: https://arstechnica.com/information-technology/2023/06/op-ed-why-the-great-twittermigration-didnt-quite-pan-out/
Either way, you won’t convert anyone by attacking them. If you want Lemmy to be able to replace proprietary social media platforms, which is something I want, you have to meet the users’ expectations. The expectation for Lemmy is a Reddit-like experience. But with the fracturing of Lemmy into instances that block each other, normal user will simply stay on Reddit.