Suppose C is starting bitcoin full node for the first time. C would have to download blockchain from some place. Could not this leave bitcoin susceptible for tampering? If blockchain stored in decentralized way or if blockchain full node constantly check their blockchain, wouldn't this still leave bitcoin susceptible to tempering? For example, if people downloaded blockchain more from a particular group of nodes intending to tamper blockchain, would not blockchain be compromised? If resolution is done based on bitcoin wealth, this also seems to leave serious issues..