In london 90% of all deloveroo/uber eats is delivered with an illegal ebike, nobody cares.

It doesn't make it legal. Anyways, adults make their own decisions and pay the consequences, I'm not here to tell people what to do. Just keep in mind that e-bikes are very regulated in a lot of European countries.

If someone's wallmart bike with a 1200w aliexpress "push button" motor end up injuring/killing someone (due to undersized brakes, snapping chain, &c.) I'm sure a great deal of people will care about them

You're acting like these Walmart bikes with 1200w motors are commonplace, and sold in mass quantities. Most people aren't going to spend their time fitting this stuff on a regular-ass bike. These "kits" don't even bolt onto most bikes, and require some ingenuity to make them fit. What I'm saying is there's not enough of them out there for people to ever care about the issues you bring up.

I think you, and damn near every other person on the internet hand wringing about this kind of stuff, vastly, vastly over estimates the conversion rate between "shitty bike with more power than it probably should have breaks something" and "mows down some elderly lady"

And you're also over-estimating the number of people who will care when that conversion happens. Because odds are when someone does have a mechanical failure and mow down an elderly lady it will be preceded by a bunch of stupid decisions not having anything to do with that mechanical failure and contrary to what you may believe based on HN/Reddit/Twitter commentary, the general populace is well aware that you can't legislate away stupid.

And as others have said, there's a mechanical aptitude bar to entry for using those kits that make them less common than you're implying they are.

Well arguably the fault lies with Deliveroo because they profit from illegal activity, but pretend its not their employees. They know what is happening and tacitly encourage it.

If i understand bike law correctly, for offroad biking you can use anything, but ofcourae if you rig together something stupidly dangerous and cause an accident, a court will take dim view of it.