SDI is probably the main remaining prevalent signal being used with BNC connectors, and 12G-SDI is going to be its last hurrah. 25G/100G Ethernet and is rapidly displacing it.
The main thing SDI & coax has going for it is its robustness, especially for 3G-SDI signals, so I guess it will live on there. But BNC & SDI is on the way out, certainly for broadcast. Its so much cheaper to use Mellanox, 25G/100G Ethernet instead.
I'd assume it will be decades until BNC/TNC die off completely. I replaced one today on a Bell 205A1 helicopter, for the ELT. Given the standards for reliability (both in how well it must perform and how long it must last) aircraft OEMs aim for, I expect to work with them for the remainder of my career.
The main thing SDI & coax has going for it is its robustness, especially for 3G-SDI signals, so I guess it will live on there. But BNC & SDI is on the way out, certainly for broadcast. Its so much cheaper to use Mellanox, 25G/100G Ethernet instead.