'maintained its independence' is perfectly correct and usable in written English.
You can use it when you want to express that something (a person, a country, or an organization) managed to remain autonomous or free from external control. For example: "The small island nation of Tuvalu has managed to maintain its independence despite being surrounded by powerful neighbor countries."
Before this, Quiriguá had been a vassal state of Copán, but it maintained its independence afterwards.
Notre Dame football is big business and a national brand, but it has maintained its independence in an era of sprawling football conferences.
Efforts to unite Inner and Outer Mongolia failed, and Inner Mongolia remained a part of China while Outer Mongolia (now Mongolia) maintained its independence, though it was a client state of the Soviet Union until the early 1990s.
In the 1980s, it became an Italian national institute but has largely maintained its independence.
But there's a Wheelers (no apostrophe) in Whitstable that has maintained its independence.
Of these, only Luxembourg has maintained its independence, and its grand duke.
So while the media could take a more active role in highlighting those companies that are making a difference, what is vital is that it maintains its independence to do what it does best; hold power to account, challenge the status quo and give a voice to the voiceless.
Last year's Sunflower Movement, or three weeks of student-led demonstrations against an agreement for more liberal trade between China and Taiwan, underscore the anxiety many people have about how Taiwan will maintain its independence as it tries to build stronger political and economic ties with its much larger neighbor.
While Zynga is still tied to Facebook for now, the gaming company has made it abundantly clear that it still plans to maintain its independence as a business, through its Zynga Live project and other avenues.
This first process was terminated by the Del Monte board after it decided to maintain its independence.
He promised that the principles of the Manchester Guardian would be upheld by maintaining its independence and running it for public benefit, not private gain.
