RTMP is very old as a protocol and pretty rubbish.
But not because it's old. IP is old. UDP is old. RTP is old.
Using TCP for live streaming isn't a good thing, same as using TCP for a VPN isn't good.
SRT isn't great, it grew from a file transfer program (UDT). UDP file transfers like UDP and Signiant were popular 15 years ago when TCP window scaling and buffers were more miss than hit. Companies like signiant and filecatalyst were very big on them, and UDT was an attempt to standardise it.
RIST is technically a far nicer protocol than SRT, but suffered massively from the talking-shop development. It came out two years late, and even then didn't have the proper libraries to just drop into your prosumer camera like SRT did.
N.B. It's a shame as their file transfer management systems were good (control, monitoring, security, scheduling etc), but they were going full pelt on the "TCP is slow, UDP is fast" line - to the point that they deliberatly took standard linux distributions like ubuntu and redhat, and changed the default settings to disable tcp window scaling, so show a "side by side" comparison on their sales booths at events.
