Once you go up the first step of a ladder, the second step is now easier. It's a shifting of the Overton window. You can't enact full surveillance in one move - no-one would accept that. You can boil the frog over a few decades, though.

Potentially. The existence of the law exemplifies the idea the law is there to protect someone; all you need then to upgrade it is to argue that the current method is insufficient protection. Spirit of the law has been established - letter of the law will follow.

Seems so - it builds in the verification infrastructure requirement, making it dead simple to change whether it is opt in or opt out at any time.

It's actually pretty hard not to have a change be part of a slippery slope. It requires including blocks for further behavior as any subset implementation is hard to sell as not being a slippery slope path otherwise.