The cost of a 51% attack has been estimated to be $6–$20 billion [1]. This is only 1.3%–4.4% of Ethereum's current market cap. And it is only 0.5%–1.5% of its potential growth if Ethereum knocks down Bitcoin and conquers its full share of the cryptocurrency market.
This really doesn't make any sense.
The cost of a 51% attack should be measured in compute, not dollars. Let's say it was $20 billion, using a percentage valuation of market cap is not a binary calculation. You can't just sell 4.4% of the total supply of Ethereum at a fixed price to fund a 51% attack. Also, 4.4% of Ethereum in circulation is not owned by a single person or entity, that would be able to make this sale and fund this attack. Your theory is founded on a lot of assumptions and mistruths.
What I am imagining is that the Ethereum stakeholders could first of all start being vocal about this security risk of Bitcoin. This wouldn't require any effort, really, and no risk. And already it might make some investors migrate from Bitcoin to Ethereum, which would thus grow their assets. In fact, I think they are bound to do this at some point, since it could potentially gain them a lot with almost no effort.
I've mentioned that once this migration has started, it might cause a positive feedback loop (due to lower mining rewards, which in turns makes the 51% attack even more affordable), causing more and more investors to follow suit. But let's assume that this talk alone won't exactly cause a total crash of Bitcoin yet (which is of course a reasonable assumption).
But now the Ethereum stakeholders/investors are in a situation where the more they talk about how they might
actually consider an attack in some near future, the more the Bitcoin investors might get uneasy and start moving their assets to Ethereum instead. So while they might be hesitant to come out in support of an attack at first, with up to around a
trillion USD on the line, they are
bound, in my opinion, to be more and more bold in terms of being publicly open towards participating in an attack.
They might also very well invest some effort into drumming up public support for an attack, using the argument that the elimination of PoW would end up sparing the world from a large electricity consumption. Given how a very large part of the public is very mindful of energy consumption and CO
2 emissions, I don't think that will have a hard time at all swaying a large part of the public.
And after this whole (long) prelude, if the Bitcoin investors haven't already started to abandon ship, but stubbornly maintains that Bitcoin isn't at risk, and/or that a 51% attack wouldn't cause much harm to it, then the Ethereum investors might very well slowly start to band together to fund some initial acts of showcasing their power, e.g. by making what I refer to as 'partial attacks' in my preprint.
And if somehow the Bitcoin investors still cling to their blockchain, and also won't commit to try to hard-fork to a PoS consensus mechanism in order to mitigate the attack vector, well, then who knows, some of the Ethereum stakeholders might then even be bold enough to try to fund (perhaps anonymously) an actual 51% attack against Bitcoin.
Long story short: The whole process might take several months or years in total, and might include a large portion of the Ethereum investors in the end.