Today any letter received from Government in Hindi is not understood by majority of Hindi knowing people because of its difficult vocabulary. So, it is necessary to replace it with Hindustani to make it comprehensible to all.
Hindustani has borrowed words heavily from other languages, particularly English and Urdu which are understood by all. In fact, Indonesian people were more intelligent than us. They were the slaves of Dutch people from whom they gained Independence but did not remain dependent upon their Dutch language.
They established a new language Indonesian which contained about 10,000 Dutch words apart from words from Sanskrit, Arabic, Persian and other languages belonging to Indonesia. Thus Indonesian was developed as a language understood by majority of Indonesian people.
If we want a language to be understood by a majority of Indians, then we must go for Hindustani containing various English words like Uncle, Auntie, Manager, Managing Director, etc. as well ass words from other Indian languages like Tamil, Bengali, Punjabi, etc.
The script of the language, Hindustani may remain Devnagari script but it must accept commonly used words from all the languages.