Putting up cameras does shit with crime when it's managed by one central agency without crowdsourcing the effort, yes. It actually takes a lot of effort to go through false positives and all the footage, the sort of effort only the people who've been personally affected put into it, and even if you identify the portion where the culprit appears, that alone is not usually enough to identify them. It is still effective at identifying that a crime took place and to begin to define a profile of the culprit, and there are plenty of examples that prove how effective it from recordings on YouTube in countries where it is allowed. If it wasn't, retail stores wouldn't be putting them up.

You are also miscomprehending the GDPR and recital 50, which refers to things like phone recordings you take, not security cam, which you aren't allowed to put and share in social networks under many circumstances but which is generally not enforced because random passerbys don't normally sue for breach of it, although you are allowed to retain and share with law enforcement. GDPR is even criticized for its SLAPP potential on journalists.

Your take about GDPR allowing you to put up cameras is really wrong, and just about any simple search about putting up cameras and the GDPR will disprove it. If anyone really believes it, they will risk fines if neighbors or police want to be assholes (assuming you aren't trying to be one yourself). It's a shame you decided to weigh in in such an issue in a way that disinformed readers to such an extent.