You may need no Bitcoins, but you need wealth. You can't plan to spend billions of dollars worth of ASICs and electricity and retain their work until further notice (?), without damaging your own wealth. Besides, what could they do on a PoS crypto? Buy half of its units? They can't attack everything and that makes the 51% attack of Bitcoin meaningless.
People sometimes overestimate the cost for this, they take new gear at resale value, they think in power at residential tariffs and so on..
To launch a 51% you would need roughly 1 million s19pro, that would be 7-8 billion $.
But if I would do that I wouldn't go for that gear, I would grab s9s that sell for around 250$, and instead of paying 8k for 110th/s I would get away with 1500$. So rather than 8 billion, you're down to 1 and a half. And there is a ton of cheap useless for ROI gear sitting in China with no buyer, you're not going to make a profit with it but you'll definitely get a far better price per TH than with the newest generation. Besides, you don't need it to run for a year, one week and it's enough.
The other thing is energy, the first part is quite amusing, every time when it's about bitcoin, everyone says bitcoin is using only 0.01% of the energy in the world, now when you need to attack it, god that's a lot of energy and nobody can get it!!!! Back to the math, miners can't spend more on energy than the reward, so to match them you will need to spend a max of 45 million a day, but you're the government so no taxes, it will get way cheaper. If we go by 10cents/kwh it will still only cost 20 millions to power 6 million s9 a day.
Now, let's assume that indeed Bitcoin is threatening the US dollar, and the US who spends 13 billion on a Ford-class carrier won't be able to pull that off?
But nobody is interested to do so, why do it? It means people in your own country losing wealth, it means stopping an activity and dealing with people that once had a job and now are on the unemployment payroll, why? Because it will replace the dollar and it will do that and that? Common...
Seriously, think it. Do you really believe that after this revolutionary creation, people will simply forget it? That this innovative technology will be gone just because the government says so?
Once the value is gone so is the hype. The idea for which Bitcoin was created get's more tainted every day, look at the ecosystem now, it's all about centralized exchanges and wallets, prices and trading, and holding till you make x in profit, once that is gone you will never be able to find the same userbase again, the ones that are reticent to the idea will never embrace it after such a failure, and from their point of view is quite understandable.