Even if they know which address belongs to you, as long as you don't need paper money, they can't do anything. They can't take it away, seize or steal.
Depends on who "they" are. Thugs can take your bitcoins at gunpoint. Government can do essentially the same thing, disguised as "taxes".
As long as you stay in btc, taxes make no sense. Also, You can always say "that address doesn't belong to me anymore" or "oh shit i lost my privates sooorry :d"
If a country taxes you before you sell your crypto for FIAT, that country is already in deep shit. You shouldn't ever buy coins through banks anyway. Only mining or cash/local btc buys...
About the thug thing, that's a different situation. I mean unless you are dumb enough to share your balance with unknown people, they won't be able to know it. However, the government can see where your money went to because banks will share information and people usually buy btc via banks. In the end, if you stay in btc, you'll be in a
relatively safer position.