Even if we all agree with you that governments should not store millions of BTC, who is going to stop them if they want to, the BTC network is decentralized and censorship resistant, so anyone and any institution is free to accumulate as they like. However, i don't think BTC will be popular amongst countries, neither will they desire to have BTC in millions as part of their reserves, most of what is said in the U.S these days is just campaign strategy.
- The US buys 1-2 million BTC, other states/central banks like the ECB, China and India follow, and together they hold let's say 5-7 million (about 25%-35% of all existing Bitcoins).
So if we hypothetically agree that different governments accumulate this much, it would still be almost impossible for all of them to collaborate to 'hurt' the network, they don't collaborate on so many things, so why this?