I think at this point anyone that is interested enough in crypto to be invested (or to consider investing) is already well aware of the PoW vs PoS situation.
Regardless of that it seems impossible to guess how the market would react in practice, but given that history suggests both that (1) Bitcoin crashing takes the rest of the market with it and that (2) markets react surprisingly little to 51% attacks (though those were admittedly rather minor alts) it seems like the odds are not especially in an attacker's favor -- at least in the scenario of Ethereum trying to take Bitcoin's throne by means of a 51% attack.
Yes, we certainly agree that it would likely require some change in the public perception of crypto. But who knows, that could happen. And it seems to me that once the Ethreum stakeholders truly realize that there could be up to a trillion dollars on the line, they will have more than enough motivation (and means) to make the public more acutely aware of the competition between the two blockchains.
According to the paper the range of 6-20 billion would apply to an attack that lasts for one hour. So that's about 6 blocks, not hundreds.
No, the operational costs that the paper mentions are only in the millions. (And if we consider the daily total mining rewards, we also see that the price is measured in the millions.) So by far the largest part of the price of a 51% attack will be due to acquiring the ASICs (and/or compensating bribed miners for their ASICs).