That's not how it works. A country making Bitcoin a legal tender doesn't get control over it because they don't change anything, it simply makes Bitcoin accepted and allowed within the country, The least it can do is put some regulations on its usage like taxing transactions in some way, not allowing people to have or hold more than a certain amount in it, etc., but they can't control Bitcoin and make it centralized or anything.

Do you think that a government making Bitcoin a legal tender can have control over the blockchain and the transactions being made, etc.? No, they can't control that, all they can do is track certain transactions that are available online in the blockchain for anyone to see.