I think DLNA never really did "casting" as we know it, as another commenter says, it was a file browser and you could send file-based content, but you didn't have it just showing video or audio streams directly, ie cast my actual screen.Isn't this what DLNA is supposed to have done for the past 20 years? Maybe people have figured out better ways to do this since then, and it's easier to make something new than fix DLNA which doesn't work the best, but maybe not.
I looked it up, and DLNA was dissolved in 2017, probably to little or no fanfare, it was limited in utility.
Miracast (Wi-Fi Association standard) was a better implementation of open standard casting, but it's fallen out of favor too.
RCS is basically a replacement for SMS and MMS and it's badly needed. MMS and SMS are ancient protocols and need to be updated, so it's more like the whole world forced Apple's hand, from Google to Microsoft to even the GSM Association.Forgive my cynicism, but I feel like Apple is only implementing the RCS as a means of staving off accusations of anti-competitiveness. I wouldn't hold your breath waiting for them to implement another protocol (if they even can... this is proprietary I'd assume).
It'd be like Apple supporting 802.11b only and acting like Wifi (standards-based text messages) is outdated in the world of cellular (iMessage). Yes, that's an imperfect analogy.
But still, Apple can go either way on Matter casting. They already are supporting Matter so why not, on the other hand they really like Airplay. But it's a pain when you buy a fancy TV or smart speaker and it supports Google Cast but not Airplay. I think Apple wants to avoid a similar situation to Homekit being widely unsupported so they added Matter to try to fix this issue.