What’s preventing streamers from streaming to multiple platforms at once. Is it against he Eula or something ?
Since the communication direct is still mostly one way, you can e.g. aggregate all the chats from multiple platforms into a single one, and treat it like you would right now. Rocketbeans.tv, a German television-like streaming station that I watch a lot has a "superchat"[0] that does just that (which I think they built in-house), and it works really well. From a quick search, it looks like Restream.io also provides such a tool.