But why do people tell me, that a blacklisted Phone, that goes online once, will be locked in a way that it will never be able to be activated again? Great Britain and Germany support this service for example.
The carriers do it in those countries only because they are required to by laws in those countries.
The carriers don't care if a phone is stolen. The original owner will buy a new one and continue to provide income to the carrier (even it the phone isn't replaced, if on contract, the original owner will still have to pay monthly fees or the early termination fee), and the user of the stolen will pay some carrier for service. The carriers end up with more money since they now have as customers some people who could not afford a new phone but can afford a stolen phone.
The carriers oppose laws blocking stolen phone because they end up with less total revenue, and have to go to the trouble of keeping lists of stolen phones.