...because the existing and established ones have something unique to offer and to copy that would be an enormous (and slow) undertaking (but it is being done, civilization has always been spreading to free areas, it is just slow) - but once you crack open an hassle free secure consensus system (software, not bound by limited physical reality) the old one does not have anything unique to offer for the group of people who are unhappy about who holds the power.
If there was an exact replica of Manhattan (or if it was trivial to create it) - and I mean really exact with all the affordances it provides - think of an empty copy of the world, Manhattan is in the same place, but mostly empty (just established), that you can switch to with a keypress (and could switch back and forth) - what would happen?
>...because the existing and established ones have something unique to offer and to copy that would be an enormous (and slow) undertaking
Exactly. That's also the case for cryptocurrencies. You can instantly fork a cryptocurrency or a blockchain, but you're essentially copying a blueprint. You can copy Manhattan's city layout as a blueprint or other written/drawn representation, but you still need actual builders and resources to copy the physical structures, just like you need miners, mining rigs, and users to build an alternative to Bitcoin that has any value.
Assuming the same PoW algorithm, you'd need your alternative to exist for as many years as Bitcoin does to achieve the same blockchain length, for example, with a lot of distributed miners to achieve similar security guarantees.
You can create a Hacker News clone pretty quickly. (For the sake of argument, let's say you could do it in a day by forking an open source repo.) It doesn't mean your clone instantly devalues Hacker News.
Maybe if you grow your clone's community over years and it generates a lot of interesting discussions, people will use both, and maybe if eventually the discussions become more frequent and have higher signal:noise than HN's, HN activity may gradually decline as people decide to use your site more than HN. But this is about community and network effects, which you can't copy overnight. You have to build them over time.
If there was an exact replica of Manhattan (or if it was trivial to create it) - and I mean really exact with all the affordances it provides - think of an empty copy of the world, Manhattan is in the same place, but mostly empty (just established), that you can switch to with a keypress (and could switch back and forth) - what would happen?