" Because streamers want to cater to the widest possible audience, they'll often opt for a low 720p60 resolution. "
My understanding is that 720p60 isn't about reaching a wider audience, as the streamer is likely going to max out their bitrate regardless of resolution. I think 720p is used due to a lower impact to the encoding PC, as well as the belief that 720p60 looks the same or better than 1080p60 at Twitch's low 6mbps maximum bitrates compared to YouTube letting you send them >50mbps.
As for why the 5 concurrent stream thing is Nvidia news, I'm unsure. The 2 encoders bit isn't news, and I thought Nvidia lifted their (easily bypassed) Encoding session restriction to 5, like, a year ago. As far as I know, their competitors are still unlimited (but AMD's encoding quality is still lacking).
I guess the new thing is that Twitch will let you send them multiple streams at once? I feel like it should be cheaper for Twitch to just transcode down the higher quality stream compared to putting that burden on it's employees/users and buying all that extra bandwidth. Does the bandwidth going toward lower quality streams get deducted from what you can put toward your regular stream? YouTube goes with the transcode option, at least.
Also, did Twitch massively raise their bandwidth cap for users? Because if not, that "4k60" option on the slide that goes with this is a complete laugh. Because 4k60 at 6000kbps? Come on, are they even letting people try that? If so, why?