That is a lot of assumptions about an attacker's means and willingness to invest in the attack. German cryptographers made that same mistake during World War II: they knew how to attack the Enigma cipher in principle, but assumed, incorrectly, that none of the allied powers would be willing to make the necessary investment. Tens of billions dollars is within reach for many governments, corporations, and even some individual people. It is not hard to imagine what might motivate an attack, whether politically (perhaps to disrupt the Western financial system once more institutional investors have take on large Bitcoin positions) or financially (imagine building a large short position in Bitcoin futures, then executing a 51% attack).

Moreover, why assume that the attacker will necessarily have to work as hard as the total work being done by honest miners, when the attacker might first try to take down the largest miners as a first step to reduce the capital investment? Blackouts in China significantly reduced the hashrate of several large mining pools recently. An attacker willing to invest tens of billions of dollars in an attack would have little difficulty sabotaging mining operations, especially given how concentrated and centralized those operations have become.

Really though, in what other context would we accept an argument that comes down to assumptions about what an attacker is willing to do? Cryptographic security demands that the attack effort grows exponentially (or super-polynomial in the case of public key cryptography) so that we can make the attack effort larger than the number of atoms on our planet (or galaxy or universe), which renders all speculation on the attacker's motives or willingness to invest an attack moot. With Bitcoin you can never make such a claim until most of the world's available energy is consumed by honest miners (the only point at which the attack is actually impossible, at least if we assume that the honest parties cannot be corrupted).