The thing is, tech wise Common Lisp is in very good shape. It could very much take C++'s place and everyone would be happier and probably get similar performance with cleaner code.
Likewise, Scsh shows that a lisp-1 could encroach on territory typically occupied by Ruby and Perl.
These aren't Lisp's problems. Lisp's problems are social. It has no anchor point, and most of the modern, successful languages have an anchor point either in a tightly knit community, a corporate backer, or a charismatic personality. pg could assume this roll, but thus far he hasn't opened up the arc development enough to really allow that to happen. Given his duties at Y-combinator, I guess this is understandable, but it's not helping Arc at all.
Arc seems pretty open to me. There's nothing stopping people from creating countless useful libraries. I'm guessing that many people have turned away from Arc after seeing the dirth of libraries. Paradoxically, that's one of the things that attracts me to Arc. There are many opportunities to make an early impact within the Arc community.