Slippery slope is not a "fallacy" by default. It can be occasionally but its a perfectly reasonably argument in plenty of cases.

What "constructive" argument is anyone supposed to give about authorities having warrantless access to all private conversations?

Constructive argument? Just disband the EU as a whole, including all laws, treaties, contracts ...

Europe would be a much better place if the EU stayed what it was, a trade union of sovereign nations without any political power over the people.

"Slippery slope" does not by itself invalidate an argument, because slippery slopes do exist.