Ok I think you're new to Bitcoin and economics, lets go through some things.
First off, there are not currently 21m Bitcoin, there are about 19.1m or so. In about 120 years there will be 21m Bitcoin and there will never be any more.
Ok so if govts decided they wanted to 'destroy' Bitcoin by buying it all up so nobody can use it. Well the price of anything is a function of supply and demand. Bitcoin's supply is limited to 21m, and if governments decided to buy every Bitcoin in existence, that means demand would skyrocket exponentially. Because the government would have to convince every single owner of Bitcoin to sell ALL of their Bitcoin. And the only way to convince them would be to offer higher and higher prices for that Bitcoin.
Let's pretend that all 21m Bitcoin were in circulation now (instead of taking more than a century from now for that to happen) so that no new Bitcoin will be mined after the government tries to get them all. Well there's also probably 2 million or so Bitcoin that are lost so really the government would have to only buy maybe 19m even if all 21m were already in circulation. So the current price is $20,000. What would happen is that governments would have to bid the price up into the millions of dollars. This would cost governments likely tens of trillions of dollars. And at all times they would have to outbid all the other buyers in the market at all times. So when other people buy Bitcoin the government would then have to offer them even more money for those people to then sell their Bitcoin for a profit. The governments of the world don't have tens of trillions of dollars, and certainly not tens of trillions of dollars to spend on something that has nothing at all to do with the operation of their nations.
Secondly there would likely be people who would refuse to sell, seeing as how the government just keeps raising the price of Bitcoin in their attempt to buy it all. And there would be no way for the government to know whether some of the older bitcoin that hasn't moved in many years is lost or is just owned by people who won't sell it. But let's just say the government does manage to buy all the Bitcoin in circulation, and they've had to massively inflate their own national currencies in order to print enough money to do this, of course raising the price of Bitcoin even further at the same time they are doing this because their national currencies are being devalued. Given the devaluing of their currencies, maybe they end up spending hundred of trillions of dollars to complete this, they've crashed the value of all national currencies so that massive economic pain hits all countries in the world and the only people that still have enough dollars to be well off are those who were already very wealthy or those who had a decent amount of Bitcoin that they sold to the government. Everyone else in the world is in financial ruin. Economies collapse everywhere. These very governments that did this are then overthrown all around the world because they completely collapsed their economies and they can't even afford to fund their militaries and police forces and other security forces. The world is thrown into a lawless, anarchy with an economic collapse that makes the great depression look like a nice time.....and all so the governments could buy up all the Bitcoin that are now completely worthless because they are the only owner and their whole plan was to be the only Bitcoin owners so there is no one to transact with and they don't want to sell it to anyone. Great job governments, you just destroyed all of society simply to try to stop Bitcoin.
That's what it would take and roughly what would happen.
The far easier way for governments to try to stop Bitcoin would be to make it illegal or make the regulations around it so cumbersome that it is hard for people to get their hands on and hard for companies in the space to operate. But of course all governments aren't going to do this at once. So you'd end up having some governments implement this monetary tyranny on their people while other nations would see this as a once in a lifetime opportunity for their nation and they'd open up their country and laws for Bitcoiners from other countries to come live and work in these places. So you'd get tens of millions of people (some of them very rich) moving from monetarily oppressive nations to the nations that saw this opportunity, thus hurting the nations that tried to stop Bitcoin while greatly growing the economies of the nations that welcomed Bitcoin in response to the monetary oppression elsewhere.
It is never profitable in the long run for governments to try to restrict freedom in oppressive ways. Bitcoin represents the greatest monetary freedom in the world. For every government that tries to put a stop to it that will just lead people to other countries where it flourishes, hurting the places that restricted Bitcoin and helping those that opened up to it.