I would be seriously beginning to worry, were I working over at Splunk. I know the ES guys have said, over and over, that they're not gunning for Splunk customers but, as the ELK stack becomes a more mature tool, the reasons to use Splunk and pay their downright extortionate prices decrease more and more.

I'm assisting with a ~500GB/day cluster right now (with that number expected to quadruple in the next year or so), and ELK has proven to be an amazingly resilient and flexible tool.