I don't see why every country would develop it's own custom cryptocurrency.
The block ledger technology can be adopted and modified to function in various sectors.
And the creation of new cryptocurrencies does not really directly affect bitcoin or it's decentralization. Nor would it give authorities control over the digital currency market.
I mean to be completely fair, we have seen certain countries attempt to implement some form of cryptocurrency. Realistically, Bitcoin & other cryptocurrencies are going to be financially difficult to control. It really comes down to the countries opposition to the currency in general, the risk they feel by the currency (if any) & how much they are willing to spend to enforce such control.