If there were people who believed it was un-tzniyus to wear seatbelts, I would imagine there would be some Rabbanim who wouldn’t simply say “eilu va’eilu” but try and actively stop other Jews from doing something that’s dangerous and could lead to loss of life.
Simialrly, I completely understand R’ Twersky who is not content with saying “let them do metzitzah bapeh” because he believes it’s a sakanah and therefore is doing what he can to protect the health and lives of babies.