This is why coins like Ethereum Classic needs like 50k confirmations which is a 1-2 week waiting period for deposits, since blocks take like 15 seconds to mine and a 51% attack on the ETC network is relatively affordable compared to BTC/ETH, so fake chains sprout up very often and many transactions would be reversed
As soon as a 51% attack is executed, trust is the coin would plummet and so would its value, not even accounting for the fact that selling an entire market cap would zero out the coins value to begin with
