I think the level of impact that we see from established creators migrating will be dependent on what other features get added (or not). The implementation of communities would greatly offset the negative effect of a "larger pond" scenario. If categorization and search-ability is polished, then niche creators will still get found.

I think it will also depend on whether this platform starts to look more like a social media platform, or continues to be a blogging echo-chamber. Will people join and interact similarly to how they do on FB or twitter? Will your hundreds of friends/followers from other social media platforms who have no real concern or desire with creating content have a reason to make the jump? Maybe if curation rewards get a major revamp.