Meter inaccuracies are sometimes over and sometimes under. Losses are always under.

Across enough thousands of meters, you should be able to separate the effects. I suspect the hundreds of millions saved if a country switched to cheap less accurate meters would easily pay for other methods of policing/detecting theft. For example, 'radar' down the electricity cables can see how far away every switch, device and wire is. It would be pretty straightforward to put radar devices at a few places on the public network, then effectively triangulate to some house who is stealing power because when they turn stuff on and off (which the radar can see, together with the cable length), no nearby customers meter sees the increased load.