At the enthusiast end you can see on forums that a lot still did stick with Canon, but have since picked up the Sony A7r and adapter to use with their Canon lenses. This isn't statistically a very large segment though! There will be users who will pick a D800 over the 5D3 as a first purchase, but likewise the other way as well. The 5D3 has performed well in sales.
That's the thing - yes there will be first time buyers in that market and those who already have kit but who are prepared to switch but they represent a tiny percentage of the overall market and I doubt Canon is losing any sleep over it.