This is why all these discussions about "absolute consensus security" are moot.  In reality, people forget that
1)there is a market
2) there are news feeds
3) that you can't do that unnoticed (some people will have "old copies of the block chain")

What these consensus algorithms do, is to prevent some moderate-sized hacker group to overthrow the system.  But if a major player really, really wants to (think, government(s), very rich entities, ....) none of this stands, unless we scale up to ridiculous sizes.  The only way to be absolutely sure that there cannot be a PoW attack, is to have a PoW that indicates that we use, say, 80% of earth's power production.  With the remaining 20%, to run the whole economy that remains (you know, making food, building houses, ....) there's no room to do an attack.  This is what PoW would converge us to (and destroy entirely human economy) if it would be all-encompassing.  If tomorrow, we all use bitcoin for all of our monetary affairs, that's the state we'll converge to, in an unavoidable way.
And anything less is fundamentally attackable.

There's one thing people forget: after such an attack, which will be noticed, the coin is dead.  There's no point doing an attack with the hope of being rich on the chain, because the market will kill it.  And THAT's what makes that all these imperfect algorithms actually work in practice.