The thing that makes casting so appealing for me is how ubiquitous it is. It eliminates situations with guests where they would recommend a show/movie only to find out that I can’t easily play the content because it’s only available on a streaming service that the guest pays for and I do not. As long as the guest brought a device and connected it to my WiFi, it more than likely could be casted without having to install another app and/or sign up for a new service (or have the guest login with their account).
I am becoming less optimistic about it though. I just do not think that the level of ubiquity that Chromecast reached even 10 years ago will be matched with a FOSS alternative. Developers would need to incorporate it into their apps, websites, etc. or it would need to be compatible with existing solutions. I doubt Google will open Chromecast up enough so other options can be fully compatible with it. Additionally, without the backing of a major corporation, I do not see developers taking the time to make their content compatible with another casting option.