Atleast 51% attacks do not seems to be as bad as people were thinking they were going to be. It is not instant death for the coin when it happens. Looking around at the coins that have been attacked, it looks like 20-30 Million USD might have been stolen from the 51% attacks this year.
That doesnt seem like much when you compare how much was lost from other hacking attacks that used other vetors. Or when compared to other types of businesses, Just recently one bank was hacked for 13.5 million in one go.
For how many random projects that people just started up at home one day, and then they grew from there, PoW seems to be very strong in security considering how many networks there are and how little has been stolen by exploting the PoW.