The first time I know about RTSP is from streaming low bit rate audio through my real player. I even thought the R in RTSP means "Real Network". It is common in this decade for using RTSP for streaming on mobile phone however the use case is getting limited.

The history was because streaming starts from PC, as RTSP make uses of RTP/UDP, it is good for real time content. Real time means instant viewing rather than download then view. Even packet lost is ok. Just like you could tolerate some noise and ghost images for your radio and TV rescpectively. However the world has changed.

You can imagine you will try to find another source instead if the video from internet is not clear. You want clear video on your big screen. Through your FTTB, you can achieve that easily. Why do we still need to use RTSP? On my laptop, I can watch video on demand through Flash (RTMP), I can watch most video while progressive downloading (HTTP). On other digital device, like my HD decoder, I watch live broadcast through MPEG-TS stream.

It is getting harder to think of the case to use UDP, MPEG-TS is one, however with current limited command provided by RTSP (even including Record?) It is not enough for Live Streaming and it is more than enough for VOD which HTTP is more than enough.

Maybe that's something we can think about like
"fast forward", change content sampling on displaying content (easy configurable on player).
"rewind"? revise calculating the content delta (too difficult).
"nicam" to select audio channels by finding the suitable audio track to switch

Unless somehow the viewing experience is better than Slingbox or equivalent to your home PVR, RTSP would be replaced by the other technologies naturally.