"Well they knew about them but as somebody in the French police pointed out it's just not possible to assign people to follow each step a suspect takes for the rest of his life among say several hundred similar suspects."
No, the solution is that instead of using MEGA£$€ resources to intercept and scan EVERYONE's traffic, you have deep digital surveillance on each of your several hundred known suspects. It's not possible to assign a physical agent to follow each of hundreds or even thousands of such suspects, but surely for digital communications it is much easier to analyse in detail the communications of thousands of suspects in very high detail rather than high-level scanning of all traffic, which is already happening and is clearly not working. You then use your physical agents to concentrate on high-risk suspects.
Sure, this way there are still going to be some that slip through the net, but that is always going to happen short of a panopticon state. And the general public needs to be educated about the fact that 100% safety is a myth peddled by lying politicians and that life is not worth living without some level of risk.