Is the "Slipper Slope Theory", aka the "Domino Theory", a valid concept?
I say 'Yes'. Taking federal income tax, for example. It was supposed to be small and affect only the richest. Within 10+/- years it had expanded greatly beyond the original reassurances. Today, it affects way more people than originally promised, AND the rates have increased as well.
To me, this fits. While it is not a single person or group consciously plotting something big and slipping it by us one piece at a time (that would be a conspiracy theory), the end result is that something grew to be bigger than it ever was meant to be one piece at a time.
Second example: Well-meaning politician sees that a tax or regulation was passed. They want more. They see the passage as a "mandate from the people" that what they want is really what the public wants, hence they propose taking said tax or regulation further.
What was the Dept of Homeland Security's original narrowly-focused purpose?
This is a big reason why I am reflexively worried whenever a new tax or bureaucracy is proposed. Even something small and reasonable. History tells me it won't stay small and reasonable for too long.
Opponents of the Slippery Slope argument often say that they take each issue as it comes along. I say that is easier said than done when group politics is concerned.