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
No. This means the government will regulate it. Regulating and controlling are not the same thing. For instance, if a government passes a law making certain cryptocurrency mandatory as payment methods for vendors, then it does not mean they control them.
In the previous centralized system, the government issues money and controls the monetary policy. In this new system, this power is taken away from the government and given to the public. People now have the freedom to choose which money is the best.
if this will happen Bitcoin will be the main currencies or let say the government will allow individuals to use Bitcoin for paying some goods in the groceries store then this takes to long to implement some places in some countries don't have enough high speed Internet and we all know that to transact Bitcoin is we need to have an enough internet in order to make a smooth and fast transaction. So it will be hard to those countries who have a slow internet connection. But if we don't thing about the internet and focus on the Bitcoin this still it is a good one.