The problem is that we're not dealing with just an ordinary criminal who's nicked a phone. We're dealing with a type of crime that can and does kill people. It's still not too common in the UK, but if you look at what happens in countries where it's a fairly widespread crime, it's really dangerous. They grab someone's bag and speed off while the person is still clinging onto it, often dragging them halfway down the street into traffic. They are then of course driving a heavy piece of metal at high speed through the streets, putting others at risk. So yeah, I'd say it's proportionate to take them down. I mean let's be honest, tripping a bloke who's running away from the police could technically kill him too, but we can't operate a policy of letting criminal go because one of them might die. Frankly, it's a high risk job and they knew the risks when they got into it.
As for getting the wrong person, I really can't see how that would happen. It would be massively unlucky for the police to be in pursuit of a moped robber only to find someone completely unrelated to the incident who also happens to be on a moped, who also refuses to stop. I really can't think of a scenario where that would happen.