slippery slope
The claim that one step forces a worse end.
A slippery slope says that if you allow one step, a worse end must follow. The phrase is standard. The argument is weak unless you show the steps. Do not use it as a substitute for the chain.
Examples
Use If we waive the fee, we will have to waive the next one, then the late charge.
Avoid If we waive the fee, it is a slippery slope.