Is there any PaaS for that? or need to chain multis blocks/open source projects together to "make it work"?
You could think of them as Lego bricks, combine a couple of them together and you could easily have a "broadcast grade" workflow for your content.
But back to your original question, currently MediaLive does not say it supports RTMP outputs (make sure to request it as a feature!) so using it as a "one-to-many" would require another lego brick to convert UDP to RTMP.
However, once support has been added for RTMP outputs, you could live stream to any platform (Twitch, Youtube, Facebook, Instagram) that supports the RTMP protocol.
The neat thing about MediaLive is that it allows you to use audio, video, or caption tracks. So for example, if you had a single source that had platform-specific tracks, you could create outputs for each platform and use the corresponding track to create curated content.
But, that's just one example of hundreds that is possible with AWS Media Services.
This is what Mux Video will be (source: Mux founder). If you want low-level atoms for transcoding, origin serving, delivery, security, etc. - use AWS or Zencoder + Fastly or whatever. If you don't want these atoms, but want a single API to everything "video," with decisions made by a team of video experts, use Mux.
We just opened an Early Access program if you want to check it out - https://mux.com/video.
For the higher scale video I think most solutions are proprietary - especially as most make use of CDNs and other globally deployed infrastructure. However I’ve seen startups do mass scale live streaming at pretty low latency (8 - 16) seconds by just transcoding and serving clips off of S3
Now that WebRTC is available on all major browsers it should be easier to create some of these streaming applications. Although getting everything setup is still tedious.
WebTorrent [0] is built on WebRTC and can be used to stream videos, but it doesn't handle live streaming.