Well, 1 it is google's technology so they would need cooperation from one of the least cooperative companies in the USA. I doubt Roku has any plan to add this in their books, but we will not know until it reaches testing.
If casting random videos from odd websites is your life, then you should buy a chromecast.
The different casting programs are confusing to the consumer - Miracast, Chromecast, AirPlay, DLNA!
Welcome to technology!
TheTick (aka "Moose Twit" affectionately called by chht) Roku 3 for Pokémon TV / YouTube / Animal Planet GO / HBO GO. Chromecast for most other media because Roku Mirroring sucks , how many clicks in Wndows 10 (about a dozen) to Miracast on Roku vs 2 with Chromecast. Verizon FIOS TV and Internet Service
The screen mirroring that roku provides is 'miracast' which requires a wifi-direct connection between the devices, and not all PC hardware can do that. Chromecast works over your existing network connection but the screen mirroring part does not work on the roku. Some applications provide their own 'casting' function where the phone app tells the TV app what content to play but the TV app gets its own copy. YouTube/Netflix/Plex and possibly others may work this way on the roku (you'll know because the phone app itself will show the cast icon).
Generally, screen mirroring of some other device is inefficient and not the best way to view things even where it does work. If you are playing content from the internet, chances are there is a roku app to play it directly. If you are playing from files, you can use a server like Plex on your computer to send them, or put them on a USB drive to play with the roku media player, or put them on a phone or tablet and send them with the roku app.
If you really, really need chromecast protocol, get an android TV instead of a roku or a separate chromecast device. Or, if you want a perfect display from your computer screen use an HDMI cable to the TV.
You can mirror a windows PC to the Roku with varying results - I have an HP spectre that works almost perfectly, but another PC will not. Oddly the spectre connects but is too slow to be usable with a Sony TV that also offers miracast and I have no idea what the difference is.
Anyway, this app might be your solution for the Mac if you don't want to use an HDMI cable. I haven't tried it myself but it might be worth the cost and it looks like they have a free trial so you can be sure: https://www.airbeam.tv/