Roku has not "lost proprietary control of their TV software" as they provide/update/manage both RokuOS and their own "live" TV streaming/services & content (e.g. The Roku Channel or Live TV apps).
Keep in mind C-SPAN is a non-profit group made up of constituent cable & satellite TV providers, of which Roku isnt (and nor are other OTT providers like Google's YouTube TV & Disney's Hulu+ Live TV which also dont carry C-SPAN) - each of these would have to pay license fees to carry C-SPAN within their respective app/services.
However Roku doesnt code third-party apps (e.g. a potential C-SPAN app) nor provide third-party app content (e.g. congressional coverage within a C-SPAN app) - this is entirely up to the third-party app/service provider (e.g. C-SPAN).
Again you need to contact C-SPAN about providing a RokuOS app (as they provide an Android & iOS app) - they would have to provide and manage it and (access to) its content, which of course requires TV provider (C-SPAN constituent members/licensees) login to access Live TV.
Ultimately you seem to be demanding/asking for 2 unrelated things:
- A C-SPAN app (C-SPAN must provide this, requires TV provider authentication for Live TV -contact C-SPAN about this, more likely to happen)
- Roku carriage of C-SPAN channels/content on Roku's Live TV service/app (Roku would have to pay license fees for this, since they are not a cable/satellite provider or member - contact Roku about this, less likely to happen).
The former is far more likely than the latter, since C-SPAN already provides apps for some platforms (Android/iOS) and would just be expanding their app platform reach; the latter is far less likely since it requires Roku paying substantial and significant licensing fees (contrary to claims elsewhere), which even Google and Disney arent paying for their respective OTT TV services to gain access to C-SPAN channel/content re-transmission.
If you want to watch C-SPAN on a Roku device now/today, you are going to have to subscribe to a cable/satellite TV service that has a RokuOS streaming app and access to the C-SPAN channels within it.