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.