I think you are dismissing the abandoning the power of choice argument unfairly. I'll try to give it a better formulation than you did.

There is a decision to make. The question is not just "A or B?" It is "How shall this type of choice be made?" and "Who shall make this type of choice?"

Changing how decisions of this type are made and who makes them are dangerous. "Holocaust denial is verboten," implies "There are authorities who decide what beliefs are acceptable." and "Individuals cannot be trusted to seriously consider all alternatives for belief." The second is true, because by removing the ability to reject the holocaust removes the ability to consider if the histories are reliable.

For the Civilization example. "I should decide on when to go to sleep based on a prior rule I accepted." is a different principle than "I should decide on when to go to sleep based on my current enjoyment of whatever I am doing now."

Admittedly, I rarely make arguments of the slippery slope form. I prefer to point out the toxic principles being accepted.