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.
They can never control Bitcoin, but they can regulate it to a way that it will suffocate most users. We all know that the government is crazy when it comes to big money, and all I can imagine is that they will regulate it badly, like you have to pay big taxes in order to have bitcoin, trade bitcoin, etc.
Since Bitcoin and other cryptocurrency was not made like money, they can never regulate it like the way they regulated fiat, in my opinion.