Also remember that Allied resistance in occupied Europe during WW2 was able to bomb military targets, produce submachine guns from scratch and distribute propaganda, while under heavy physical surveillance by the Nazis. Thus we know that practically any police state will not be able to stop determined fighters.
No need to steam open letters and read them physically, phone taps which can automatically surveil entire cities making connections between people, cameras able to follow people's vehicles without carefully coordinated follow teams. Who knows how the resistance would have faired under those conditions.
How about media abuse of the word "terrorism". I can imagine any decent professor at any decent university of politics to choke a little when he hears about "another terror attack".
While the definition is not agreed upon globally, in order for it to be a terrorism, the attacker needs to be sending or demanding a political message to be heard or implemented by a country he or she used violence against.
https://en.wikipedia.org/wiki/Definitions_of_terrorism
But they are pushing envelope for a well known reasons. The government has different set of rules and tools to fight "terrorism" than for example, a domestic abuse case. If things will continue the way they do, pretty soon your drunk neighbor that hit his wife will be called a "terrorist".
Aside from the fact that zero people have thus far been killed by Islamist terrorists in Germany (since...I'm not sure when), every single incident was perpetrated by one person acting entirely on his own. And the thing in Reutlingen which keeps getting lumped in was a "normal" murder.
No amount of domestic "anti-terrorism" law enforcement would have the slightest impact on any of this, not until you crank it to the level of a genuine police state with zero freedom.