As someone who works day-in and day-out with Miracast devices, you're a little bit off. The problem isn't really with features and DRM leading to incompatibility. The HDCP portion of Miracast is pretty straightforward and isn't often the problem. The problem (as seen with the charts) is manufacturers that are sorta-kinda-using Miracast but not really. For example the Samsung AllShare devices were never advertised as Miracast capable, but they happen to work (sometimes). Same with the HTC Media Link. Customers get these dongles or TVs and expect it to work with other Miracast devices, even though they aren't really Miracast. Give the standard 2014, 2015 and let's see where we are.

The Chromecast comparison is really a bad one - yes you can tabcast, but really it is designed around a "send this URL to play" experience which is much much easier to accomplish than real-time streaming. For the real-time streaming, you only have a single sink device to worry about and it only works on desktops (for now). Your surface area is much smaller, so of course interop is going to be better.