"Highly rated" is an interesting property. One of the ways that a publication venue becomes highly rated is by being highly restrictive. In fact, the primary measurement of the quality of a publication venue is the acceptance rate of that conference.
WikiSym is not considered highly rated because a high proportion of the submitted papers are accepted. Would a wiki journal be more restrictive in order to gain a "highly rated" status?
I think it's interesting to ask why WikiSym needs improvement and why attendance has been falling. If a WikiSym is a wiki conference that is struggling to maintain participation, how might a wiki journal surmount such trouble? Assuming that the answer to my question above is "yes, the wiki-journal would be more restrictive", how would such a journal gather more submissions than an established conference like WikiSym -- enough to both produce regular issues and maintain a high rejection rate?
Here's a quick point: attending a conference requires a greater committment than sending a paper. I would not send submissions to WikiSym unless I am really sure to have both time and money to invest on the trip. By contrast, a journal only expects authors to deliver an article. It does not matter where they are, and how they manage to get their work done.