[Adam Michnik]

This is not completely true and i do not think is entirely about HD/SD/3G-SDI over fiber, but Ethernet over products, especially singlemode 1310nm is as cheap as multi-mode 850nm, there a positives and negatives to using multi mode versus single mode fiber optics.
Multi-mode has less bandwidth but it does not effect SDI signal,
There is devices on the market that use unique adaptive technologies, it allows you to use either fiber you want for SDI optical transport.
SDI fiber extender https://thorbroadcast.com/sklep-240/produkt-222
It’s also possible to multiplex different signals such as Ethernet, base-band video, and RS-control(232/485) with the same HD-SDI signal. There’s a lot more to this than you thought 🙂