The best defense against truck-ramming terrorism is with simple physical barriers. There is no need to panic.

Especially there is no need to authorize increased surveillance of people in general. The Berlin truck attacker was a known known terrorist sympathizer who had been investigated and was facing deportation soon.

This specific suspicion could have been used (and, I expect, was used) to obtain orders for surveillance and searches targeted at him personally. Increases in surveillance of everyone in Germany would have provided no more information about him.

It looks like they have lots of possible suspects (only a tiny fraction of whom will actually commit any crime) that they can't keep close tabs on all of them.