@franky1, you make some very good points. In particular, it is true that the Ethereum stakeholders wouldn't just be able to buy the ASICs from one day to the other. Nor will they likely be able to bribe the existing miners unless they also pay for their ASICs as well, since these will most likely be very hard to sell after a fall of Bitcoin.

This cost is considered, however, in the referenced $6B–$20B estimate by Nuzzi et al.

I also want to contest you on the statement that Ethereum would collapse as a result of buying enough ASICs or bribing enough miners. $6B–$20B is only 1.3%–4.4% of Ethereum's market cap, so it will only require 1.3%–4.4% of all Ether in order to pay for a 51% attack. Not something that is at all likely to cause a crash of Ethereum, will you agree?

To your point about the attackers having to 'wait around to collude' (assuming that they choose to buy ASICs rather than bribe miners and buy into existing infrastructure), it is worth pointing out that they don't have to be idle as miners in that time. They can behave as honest miners right up until the attack. Also, since this will mean that rewards will become more thinly spread out, some of the actual honest miners, who aren't backed by Ethereum stakeholders, will likely fall off in that time, making it easier for the attackers to gain a majority.

And to your point about the consequences of a 51% attack, it is worth underlining that the majority of the costs of a 51% attack are from buying the equipment. The operational costs are small in comparison. Once the attackers have gained a majority (and with the backing of Ethereum's stakeholders), they will thus be able to rewrite many more than just a few blocks of the history. They could potentially steal a lot of bitcoin in doing so. And it is therefore unlikely, as far as I can see, that Bitcoin's value wouldn't be severely damaged by this. Would you agree?