This hasn't been true in many places since the takeover of digital cable in the 2000s. Without a bidirectional data link, how do you think cable internet works? Or cable telephony? Or pay-per-view for that matter? It's all on the same line into the same box.
> Cable systems don't...
Don't isn't couldn't. When talking about solutions to problems, merely describing a problem isn't where the conversation ends.
> Sending some out of order I-frame every time a channel was changed means every stream has to be decoded in real-time at the head end
That's what I described. The cost of doing it is epsilon.
> Cable is a shared medium so every channel flip by every user on a node will to be sent to everyone.
If you can stream Netflix while watching TV without hurting your neighbors, then you can receive one single I-frame.
Cable systems don't often re-encode streams they receive from upstream sources. The video from upstream is already compressed so the elementary streams are remuxed into the cable system's channel plan. The cable head end doesn't receive raw video and encode it in real-time. Even local OTA stations just get remuxed rather than re-encoded.
Sending some out of order I-frame every time a channel was changed means every stream has to be decoded in real-time at the head end in case someone flips a channel. Cable is a shared medium so every channel flip by every user on a node will to be sent to everyone.
Even IPTV isn't a bidirectional signal. It uses IP multicast and the tuners just change their multicast address to change stations. The stream is the same MPEG transport stream as the QAM signal but sent as IP packets. The head end still doesn't decode every station to send bespoke I-frames in the event someone flips the channel.