Yeah I tried to summarise that in the what would prevent fatal attacks paragraph. I try to see through a "what effective measures have we done to regulate things that can cause harm" lense.

Cars can cause harm, we demand training, licencing, registering, insurance, stoppages by police to provide proof of the above, etc etc. There are still fatalities though. I would accept every one of those measures being applied to dog ownership, you're right it sucks for the peeps owning chihuahuas, their insurance will likely be low though.

That said, after engaging with the dog foster system (as a fosterer), I have other reasons for wanting to limit access to dogs. I weigh the harm done to families that lose an opportunity Vs the harm prevention I believe the above will deliver by decreasing the number of homeless dogs and increasing the living standards by improving the skills of the owner. I would add all animals are neutered at the earliest possible convenience unless owned by a registered breeder, that registration being an absolute bastard to get, perhaps requiring yearly inspections to curb puppy mills too. I love dogs, I don't see their ownership as a right but a responsibility first and a privilege second.

I did think about banning characteristics, which then lead me on to blades as we regulate blades by characteristic. I have absolutely no idea if it was effective though, and didn't care to check, so I didn't include it. But how to apply it to dogs, weight maybe, biting force are objectively measurable but what happens if some family just lands an absolute chonk. As you say, the actual relevent characteristics are functionally impossible to police.