Technically? Nothing. Legally, this would be huge privacy incursion. In the EU, we have, among other things, (usually constitutional) right to privacy, anti-harassment and anti-stalking laws etc.

Also logistically, it is nearly impossible. The idea falls apart when attempting comprehensive surveillance. There is simply too much noise and too much uncertainty from the low quality, at distance, uneven illumination video. I write FR software, and large installations typically fall apart because the humans are simply too lazy, they delegate too much that should not be delegated, they are lax and frankly incompetent. Outside of developing tech orgs, FR is too complicated for under-educated persons like the police. At minimum, you need to understand probabilities better than a typical consumer.

How does cross-camera object tracking fare? Don't need to recognize someone if you can follow them until you see their face or their car or destination clearly.