Only you can answer that. Leave C4D only if it annoys you sufficiently to do so or if you have tried another app that feels more intuitive to you (good luck with that ! :)
If you know one bit of 3D software you have a vast advantage learning another one over someone who is new to everything. Although all the major apps have their unique ways of doing things (Blender and Modo most obviously), your knowledge of 3D language and terms will usually transfer straight over.
Z-brush is the one exception I can think of to that; its interface is just a peculiar mystery, no matter how good you are with 3D elsewhere !!
C4D is principally used for Motion Design, and as such is in not much danger of being displaced by anything except Houdini, which is hardcore difficult to learn. Of course the 2 apps play very nicely together as well so they aren't just in competition. And Maya have just added motion toolset that is vastly superior to what they had before. But I would speculate the very opposite might happen, and that people like me, who hate Autodesk's monthly bollocks may move to C4D who, for the time being at least, continue to offer a permanent license.
It is mainly MoGraph that used to be the separator, but others have caught up recently, so the gaps are smaller. In general, I would say that the advantage C4D has over everything else is that it only takes a few years to comprehensively learn - not 10 like Max or Maya ! :)
Yes. I came from Max. And fan as I will always be of 'the stack', I am totally pleased I moved to C4D. Max's interface (like Maya's!) is still a total state compared to the sleek organisational excellence and intuitiveness of C4D, and there is simply nothing more reliable, certainly not 3DS Max !
So that's my 10ps worth, but of course everyone will give you different but equally valid answers to all these questions. Predictably, most of us here are somewhat biased ;)
CBR