The author is right that usage is dropping, but that really has no bearing on whether or not it is open source. Technologies get replaced all the time regardless of the license they use.

From my experience MariaDB is not necessarily better than MySQL. The 8.x line brought many interesting features. I dream on switch to Postgres, and try every year but for my use case MySQL is still superior (100Bi+ rows for large texts, and heavy modified - I´m also space constrained - So I need data compression and the VACUUM are not good.)

The percona distribution is very good!

I'm not going to stop using it because it's not "true open source." I'm going to stop using it because there's better databases out there.