We all know bitcoin runs in a decentralized system, so making it a legal tender simply means the government will also have control over it and it system which wasn't the original motive of it
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.
That is very true, the government doesn't have the full control of this currency but they can regulate the usage of it. Do remember, no government can have full control of this currency. But the companies that are using it, can be under their government protocols when it comes to cryptocurrency.
Small countries or state can implement btc as legal tender as it is easier to manage and establish protocols that can be informed to its people.