So you mean, "other than the fact that reputable journals are the entire basis for the dissemination of new research results, the core of hiring and promotion,

So you mean, "other than the fact that reputable journals are the entire basis for the dissemination of new research results, the core of hiring and promotion, and a major aspect of funding"? The simple fact is that like any replacement system, there has to be a reason for it to exist. And for a social network, there have to be network effects in place. Fragmentation is a problem. "Oops, we ran out of VC money..." is a problem. Hell, "We got bought by Spinger" is a problem. In the very long term, they don't have to exist along side each other. In the medium term though, they do. Otherwise, it's like going from "I'm going to quit my job tomorrow with no savings!" to "I live sustainably on my off the grid farm" with no plan for how to eat in the middle.