Sorry - I guess the "slippery slope" argument is yours. Bottom line: it's a logical fallacy - we can always impeach the President who takes any power too far.
It is not always a logical fallacy.
The more steps between action A and bad end result Z makes it more a fallacy. Here there are no steps -- he has the ability, and it can be abused. Also, how far-fetched the slope is applies. Here plentiful historical evidence of past abuse exists to show that such powers will likely be abused in the future if allowed.
No slippery slope fallacy here.
we can always impeach the President who takes any power too far.
After the damage has been done, if the abuse is discovered despite the secrecy. Remember, it took a burglary to uncover the fact that COINTELPRO even existed, 15 years after its inception (and it was just a formalization of decades of previous abuses). Can't impeach a dead ex-president.