Making BTC a legal tender does not make it centralized, because it will not be controlled by the government. And that's a good thing, but they can keep a close eye on every individual and to do that there is no need for them to adopt it as legal tender.
That is the valid argument here , OP seems looking at this to be a government controlled once become a legal tender but its not because they will only allow this to circulate but not to be on their Holding so it will never be centralization .
when bitcoin became defined as legal currency instead of private property. services that buy/sell coin were no longer goods services. but instead money service businesses. and yes government regulation was able to step in by defining it as a currency instead of private property
the peer-to-peer distributed ledger network of coin ownership proof cant be controlled..
BUT the sercives and businesses operating at the gateways to the network that do financial business can be controlled
before bitcoin was legally a currency. it was treated like private property, such as trading cards and antiques private trade