I think the slippery slope has long happened and also gone away.
There are a ton of roads with "turnpike" or "pike" in their name. Some cost money [1] others are free. What's the big difference between NYC's congestion pricing and the Florida Pike?
I guess you can fight congestion pricing in order to slow the spread of toll roads but it's not the beginning of a slipper slope. Usage fees are a very old concept (price discrimination by time is pretty old as well).