I've seen UDP used for great effect in video streaming. Especially timely video streaming such as cloud gaming. When waiting a late packet is no longer useful.
RTSP is the control protocol. Some other protocol is needed for the actual audio/video streaming. That's usually RTP, these days.
RTP is a core part of WebRTC, for example.
When you're doing a video call in a web browser, you're using WebRTC, including RTP. In fact, this RTP-via-WebRTC is the only way to send UDP packets from JavaScript!
RTSP is still used by older streaming systems and hardware ecosystems that are slow to change, such as network-connected security cameras. But in newer applications, WebRTC has mostly replaced it. Of course, the QUIC effort is in part an attempt to replace WebRTC, so the wheel continues to turn!
