[imtringued]

>That's not realistic. As soon as there is some communications between the chains, the system will reconcile.

If you think that is possible then you must not have paid attention to how blockchains work. By definition there is only one chain and it is always the longest chain in the network. The shorter chain will always be discarded and all the data within it is lost. Now we have a problem. Can't everyone just create their own chain with garbage data and then win by being the biggest spammer? (also known as 51% attack). This is why proof of work is necessary. It is basically a cryptographic lottery that ensures that there will only be one winner every 10 minutes. Since there will never be more than 1 winner there will also never be more than one longest chain. If you decide to disconnect from the network and solve the POW offline your chain will always be much shorter than the main chain. Only if you possess 51% of the miners can you do this offline attack and control which chain becomes the new one. It is possible to rewrite past transactions as well. Just start mining 3 blocks ago and remove transactions or add new ones. However, this becomes harder and harder the older the transaction is. Finding e.g. 10 blocks takes such a huge amount of mining resources that it is infeasible if don't control at least a super majority.