Can't really add anything so belatedly, franky1's response, while not exactly the scenario I had in mind, describes just how complex it would be to even mount such an attack. The preparation, the sheer breadth of collusion -- word would get out before they could put the resources in place, and the moral Ethereum devs and nodes would ensure a quick abortion.
I'm not sure that the Ethereum devs would necessarily try to prevent something which might force Bitcoin to switch to PoS. And even if a majority of them will want to abort/revert an attack, doing so will undermine Ethereum's own purpose, since Ethereum has no agreed-upon obligation to save Bitcoin in case of an attack. So even though they could abort/revert a smart contract rewarding a Goldfinger attack
if a majority of the stakeholders also agree (otherwise the devs are powerless), unless they are required to do so by law, it would undermine the freedom of Ethereum.
Furthermore, since the non-participating stakeholders also stands to potentially grow their assets by perhaps as much as 100% or more, it's hard to imagine that they would actively
vote for a soft fork/hard fork preventing this.