Ok, so how would this 'trial' work here?
The CPS don't think there's enough evidence to make a conviction realistic. This covers everything from some evidence but not enough to no evidence at all to a case they believe has no legal merit. This is going to put their barrister in a rather delicate position when it comes to presenting the case to the court, since they're not allowed to lie. You don't think the defence would absolutely fall all over that?
The problem you have here is that you want the evidence and the CPS assessment of it to be different to what it is now. If there wasn't a camera, they have to manage without one. Decisions need to be made based on the evidence as it exists, not what you'd like to exist. Our system is creaking enough without pissing away resources on extended procedural sulks because we wish there'd been video footage.
And @lolly792 made a fair point about seizures. If it was that easy, why doesn't it happen every time someone crashes a car and doesn't fancy the consequences? It isn't about hot shot lawyers, because people on MN who clearly are not hot shot lawyers came up with the idea. And the families of the victims in this case have retained their own lawyers, which most don't, meaning they're actually better placed than people in this situation usually are. So it's an important question to answer.