Techs with a central body like a CEO can't be tagged as decentralized, less they is nobody who controls the affairs of the system. However, the anonymity of telegram was misused for illicit purposes beyond scams, from the allegations on him that led to this change of terms,
telegram was used for drug trafficking, and distribution of child sexual abuse images. While it's not questionable whether it's true or not, any platform with a strong privacy like telegram had will most likely be a cove for bad actors.
Telegram is not a decentralized system, but the platform was able to centrally protect users' data from the government. It was not perfect, but it made government access to information difficult, which was good. Now, with a simple police report or request from the courts, users' information can be submitted to the government.
Criminals will always take advantage of any opportunity to carry out their nefarious actions. Even Bitcoin's decentralized nature has also been misused. Submitting the details of suspected criminals to the government is not a bad idea, but the government might use this new policy as an excuse to engage in a political witch-hunt, surveillance and invasion of privacy.