No one needs to justify the "why" when we're talking about a very well trodden slippery slope.

Fallacies don't just stop applying when it's convenient to elide justification.

That's true. However, we also don't collect history for the fun of it.

Over the course of the last 20 years, we've seen that once a data collection and digital surveillance framework is put in place, the surveillance tends to expand.

Slippery slope arguments, sans good reasoning, tend to be fallacies. However, don't fall into the trap of thinking that an argument backed by historical record is a slippery slope just because it's predicting an outcome. We might call that the "history is all slippery slopes" fallacy. Stating "this has happened before multiple times before, and each time has lead to x" is a very different argument to stating "this has happened, so the logical extrapolation is x".

if it can be that well trodden, is it really still a slippery slope?

Overwhelming empirical data across history.

That’s proof of existence of slippery slopes, not of prevelance of hypothetical slippery slopes being realized.

We’re talking about data collection and usage, not possibilities in the universe.

Exactly, which is why I'd be more interested in concrete commonalities of slippery slopes that were actually realized, and not just a general "history has plenty of slippery slopes".