Bitcoin will probably not be treated as a support of national security, simply because it takes away control from the government. Every loss of control is what weakens a government, so they will probably not accept it.
Basically Bitcoin already is the gold of the 21th century. Yes it is not accepted by many governments, but still it widely adopted. I don't think even that it is necessary that governments use it, since many (most) governments also don't accept gold payments and also don't own large amounts of it.
It doesn't really make sense that 90 % of miners would leave if I understand it correctly, since they will never leave at the same time so the difficulty will also go down which makes the remaining ones more profitable and therefore stop them from quitting.
Ok thanks everyone for the reply. It seems to be that maybe it is still important to have the possibilities for a full downloaded blockchain but for daily use it should be safe to use the electrum etc.
Because the bitcoin blockchain is getting larger and larger in size people often don't use the original bitcoin client that fully downloads the blockchain to your computer. As I understood the other wallets connect to an online server that has the blockchain on it to be able to connect with the network. Would it be possible for an attacker to upload a different blockchain history to that server so that possibly millions of users would send transactions to a wrong chain or have wrong balances displayed? Has this happened in the past or is this not possible?