Your answer can always be: Do what you can to keep me around..:)<p>On a more serious note, working with newer JS frameworks you'll always run into the issue of long term maintenance. Why do I pick jQuery (most of the time) for projects that I won't solely maintain? It's easy to hire for, it has a big enough community to support and there are a decent number of developers that know jquery (and/or can easily go from JS->jquery). I would try to move some items to jquery-like functionality as you go from prototype to production.
In my team, I'm the one that knows JavaScript the best. Everyone is asking for a help and I'm happy to answer.<p>Anyway, I somehow have this feeling that JavaScript is 2nd-level language and I'm not so proud that I'm the only one that knows it good enough. I would be much prouder if I would be "The Python guy" or, hell, let it be "The PHP guy" even...
It's indeed not wise to use technologies only one employee knows. The best way is to chose technologies together and find developers who want to learn those. You have to motivate your colleagues to jump on board. And tell the senior developer websites will be more and more depending on JavaScript. It's not 2000 anymore.