Ah, then I misunderstood the purpose of
https://github.com/CryptAxe/info/blob/master/AssumeValid.md There are still checkpoints in the blockchain parameters, but the last is from around block 300000. My first point is still the most important one - you are unable to sneak in a replacement chain without everyone noticing.
To be exact, last one was on block height 295000. See
https://github.com/bitcoin/bitcoin/blob/master/src/kernel/chainparams.cpp#L172. And yes, i agree it's impossible to replace chain without everyone notice. I'm not even sure whether all Bitcoin full node software can handle chain replacement.
Please stop pretending you don't see this as a Potential Thread to BTC and the Community we've all built!
If you didn't, why would you even post a Reply...
I lack the time/patience to try to explain this further, sorry...
I believe most people believe see this as potential (or rather theoretically possible) attack, but far from practical due to involved cost/resource and how it'll only temporarily disrupt Bitcoin network due to human intervention.