Miracast is not an alternative to Chromecast (officially called Google Cast nowadays). Miracast is a standard for wireless screen mirroring. If you play a Netflix show on your Miracast device, it plays on your phone, which then subsequently mirrors its own screen it to your Miracast display. This mirroring includes your entire phone UI and stuff like incoming notifications. At best, Miracast is an open standard alternative to Apple Airplay.

Chromecast is a standard for remote controlling video and audio devices. When you play a Netflix stream on a Chromecast display, you send it a video URL that the Chromecast dongle/box/built-in device plays independently. You can then pause/stop/rewind/fast-forward it from multiple devices in your network, none of which are playing the video themselves. Chromecast is immensely more useful and practical as a result. As a small detail, Chromecast additionally supports display mirroring but that's not why so many people use it.

There are no open alternatives for Chromecast. Some Kodi remote apps create a similar experience to stream video to Kodi. But the key of Chromecast's success is not its technical sophistication, but the fact that it has been adopted by a huge number of media services in their Android/iOS client apps. The reason for this is that it's (1) built using an easily available SDK from Google and (2) Chromecast devices enforce DRM, it's not possible (anymore) to cast to uncertified third party devices.