I know about five back-end languages but if I do not use them regularly I forget about them. That's fine, because if I go back to them, I'll recover them from long term memory quickly. Plus, there are so many ways now to get help, things like type-ahead coding, Stackoverflow, etc.
My advise is to be really good in the front-end, which you are, since that is needed for every modern platform. On top of that, be really good in at least one, preferably two back-end technologies for which there is demand. If time allows, do play around with all major platforms so that you at least understand them.
I find both a strategy of specialization or generalization to be risky, that's why I try to combine them. Know a bit about all platforms, be good in one or two. It costs a lot of time, but I consider it worthwhile.