That’s not really true, lots of social movements have momentum without popular support. Prohibition, for example.
I’d say the problem is that everything’s sort of a slope, but some it depends on the gradient. Thus, you’re right, the opponents to any position always say “But that’ll lead to <extremal example thereof>” and sometimes they’re right and sometimes they aren’t.
<Godwin>First they came for the Jews and I did not speak out because I was not a Jew. Then they came for the Communists and I did not speak out because I was not a Communist. Then they came for the trade unionists and I did not speak out because I was not a trade unionist. Then they came for me and there was no one left to speak out for me.</Godwin> Now, there’s a slippery slope. I’m trying to think of something that isn’t. Maybe “Blacks and women voting? Next it’ll be flatworms!” but in fact, we stopped there. I don’t know if anyone actually said this, but they probably would have done, neh?
A well-organized activist movement pressed lawmakers until a critical mass of them had been persuaded. And then of course it was rescinded and became exemplory of radical legislation and its unintended consequences.