SDI->Thunderbolt “converters” are capture cards, they just use TB as the physical connector. Also, not all capture cards have the same feature set. Some of these cards have SDI capture and/or output, sometimes simultaneously and some with up to 8 SDI IO connectors. Some cards also do HDMI capture and/or output of the same or different feeds as the SDI signal(s).
The landscape of capture cards (not just SDI) is pretty diverse. It all depends on the kind of workflow you’re dealing with. Live productions in particular will often demand many connections, to be able to capture multiple camera angles, as well as to output to confidence monitors and/or backup recording devices, frame syncs, switchers, SDI routers and more.
Thunderbolt falls short at 4k resolutions. It is OK for 4k24 through 4k30, but for 4k60 or higher (HDR, high frame rates and not to mention 8k) you would probably want an internal PCIe card. 40gbit thunderbolt sounds like it should be fast enough, but it turns out that for PCIe traffic it tops out at 32gbit, which just isn't quite enough.
