Well, darcs clearly has enough critical mass it's not about to disappear. Beyond that, I can't see how having more developers is a plus; it might even be a bad thing, too many cooks and all.

I personally barely work on anything aside from personal projects, but that Tailor program looks like it should keep interoperability working well enough. Actually, I do work on one SVN project, and had not considered using Darcs + Tailor, but I really should look into that.