Is Ada a failed language?

People who avoid GPL in general don't understand how GPL works. They don't realize that only changes of the GPL software needs to to published on request. For instance, if they change something in the GtkAda library to make their software run then they must publish those changes on request. Their own proprietary software which links to the GPL doesn't need to be published. However, if they take a large part of the GPL software to modify it, and sell it as their own software then they have a problem. GPL was invented to prevent such abuse.

Regarding the other points, I have the impression that Ada is much easier to handle than Rust, hence Ada code is easier to maintain. Package managers are not always a blessing. Haskell's cabal hell demonstrated that the usability of package managers is questionable. As for me, I prefer the "dumb" configure - make - make install tool chain since it follows the KISS principle. In most cases it just works.

"it[Ada] failed to find its niche. "

This is plain wrong. One of Ada's favorite niche is real time embedded systems with a strong emphasis on safety and reliability. It is widely used in aerospace, railway, ...