As I understand it, nodes on the network should accept the longest branch of blocks and ignore other branches (excuse my terminology if it's not used normally – I hope it's clear what I mean).
If this is correct, I wonder what would prevent me from creating my own node by appending blocks to very old blocks. For example, these attached blocks can include transactions from my address to another of my addresses (just to make sure that the block is valid – I assume there are some restrictions that make a block valid, such as the number of Transactions or bits in a block). After probably a long time, a branch that grew out of the current branch would have reversed all transactions from the block my node started with to the currently active block and replaced all of them with my useless transactions instead.
What is stopping me? I know I still have to mine these blocks and understand that this would probably be the main thing that prevents me from doing this now.
As far as I know, the difficulty of mining is determined by the number of blocks mined in a given period of time. Assume that one day the BTC network becomes unusable for some reason and almost no new blocks have been created. Would it then be possible to do this (after a long time to keep the difficulty of my own mined blocks low)?
Thanks for all the answers in advance. If BTC is ever going to be out of date and unused, it would be cool to magically delete everything (if there are still nodes nearby).