I feel that the large number of users is a problem, not an asset. What makes a platform good is the engagement level of the users, not the volume. A user who does not want to engage enough to create an account is not likely to be engaged enough to add significant value.
I moved away from Reddit because I don't want to be part of one monolithic site, I want to be engaged with a smaller group that has more creative energy. There is no exclusivity clause that prevents people from using both sites and accessing all the content, but having them federated will lead to homogonisation and ultimately destroy what makes this site different. To extend the milk metaphor, we are the cream, mixing us in with the milk will make it richer, but destroy us.