One basic rule for limit-making is that the burden ofproof should fall on those who want change especially since the changers often want to keep on changing indefinitely: cutting the abortion limit by a fortnight was, for some, merely an interim goal on the way to a tighter squeeze or an outright ban.
The best proof is that we now look back at both the Civil Rights Act of 1964 and the filibuster rule change of 1975 and wonder that they once seemed so impossible.