I'll throw out an idea that seems simple to me...

An *opt-in* option to require that lines on your account can not be moved to a new SIM unless the current SIM is offline as far as the cell grid is concerned.

This could even be made into something that customer service could be blocked from overriding.

If someone steals your phone, they try to get it into airplane mode as fast as possible to avoid activation locks. If you drop your phone in the ocean or off the side of a cliff, it's probably not going to remain working for long. If you're concerned about losing it somewhere where it'd remain active but you'd never find it, then don't opt in to this.