I greatly prefer the occasional original post with actuall engagement to a flood of reposts. If I want reddit posts, I will go on reddit (and also get comments and things like that). On a more meta level, the only way a community can gain and retain users is by offering unique content. If 99% of content is just content from Reddit (with missing comments), why should anyone bother to use Lemmy?
Subscribing to a comunity does not cost anything, very few people will leave a community because it is dead, but people will leave a community that spams their feed.