>a 51% attack is practically impossible today It appears you think a 51% attack is when an external agent attempts to out hash the network. This is only one t

>a 51% attack is practically impossible today It appears you think a 51% attack is when an external agent attempts to out hash the network. This is only one type of 51% attack. A 51% attack means there is a almost equal amount of hash rate being applied to two divergent, consensus equivalent chains. This can happen in above scenario or when existing miners attempt to reorg blocks (See ETC 51% attack) There is nothing stopping this behavior from occurring at any point in time. The only check on this behavior is the cost to rewrite history, decrease this cost, and you increase the profitability of such attacks.