The defense is the attack itself, it is energy expensive and resource expensive so anyone who tries to do it better have a lot of money and resources because they're gonna be needing a lot. Plus, I don't think that any country will even try to do it anyways because what's it gonna isn't really going to affect long term.
Let's say cap is $1trillion, so you would have to have north of $500B just to play the game, ... and for what? WHat purpose? These kinds of losses are stuff that start wars between nations.
I think the problem is that minions have no feeble understanding for these enormous amounts of wealth, that exceed almost all USA corporations.
Recently Musk played BTc, he bought $1.5B with borrowed money, and tried to keep his baby TSLA afloat, but to what end could have Musk done a 51%, he would need 350 x $1.5B of financial influence to play +51%
On the other hand, unlike USA where its herding cats form of GOV by stick&treat, in CHINA its one collective 'jump' and if the CCP ordered, even a simple SW upgrade to bitmain ant-miners, they could alter the course of BTC forever.
But so what, the CCP loves wealth as much as anybody else in CHINA, so why not just keep playing the game, and accumulating more wealth?
51% attack, is like the old game of 'battle ship' kids used to play, one kid would knock another kid out, and say 'where did you get that destroyer?", to which the other replied same place you got your fleet of nuclear submarines, sure its a childs mind game to talk about 51% because, especially with no understanding of the infrastructure, anything can be pulled out of ones arse, when talking this game