> not on whether the current proponents happen to have ulterior motives.
Even if the current proponents have no ulterior motive, and in fact live and die having done nothing negative with such power, it does not stop the next group in power from extending and abusing power, don't base laws on temporary trust of politicians.
This suffers from the "proves too much" problem. You are essentially arguing that government should only be given powers that cannot possibly ever be abused or be extended to where they can be abused. It is only an argument against government powers against CSAM because it is an argument against pretty much all government powers.
Arguments that have any chance of persuading a government not to do something (and the public not to support the government in doing that thing) have to a lot more specific than that.