Absolutely agree.
The market for people streaming local content at higher bandwidth than 100Mbps is very slim, and if you deducts the pirates I believe it approximates zero.
So LG probably decided to skip gigabit in the believing that those watching high bandwidth content would do so using blue ray.
But the original question about USB2 Ethernet dongles is interesting. USB2 at 400Mbps would be more than enough, and we have all seen devices with “secret” drivers. Like for instance the Chromecast with USB2ethernet support.
I unfortunately only have a 100Mbps usb dongle and a thunderbolt. But if anybody has a gigabit USB dongle please try it out and report back.