Maybe it depends on the region; in high density highly-developed regions it's one story, over rural, or desert it's another. And then if you have different rules for different circumstances, how does the pilot know what to look for? Dunno, I just put the question on the table to chew over.
and in cities with a whole range of lights at different levels, different tints, colours and intensity ditinginguising one object from another can prove difficult also, hence why High intensity lights are used to provide visual cues to obstructions.
this isn't rocket surgery.. over decades and incidents and accidents, and like i said tombstone progression, shoving a light on an obstruction provides an additional layer of safety margin.
we don need to re-invent the wheen, look at GPS mapping programming and updates, big cost solutions which can be resolved by a simple process.
obstruction tough to see; shove a light on it.
a tower crane costs i should think upwards of of a couple of hundred thousand pounds.. a couple of hundred quid in lighting isn't going flatline the economy.